/* ── Variables ── */
        :root {
            --primary: #0b1a2a;
            --primary-light: #1a3450;
            --secondary: #1e88b3;
            --secondary-hover: #2a9fd6;
            --accent: #5fc3e4;
            --text-dark: #1a2634;
            --bg-light: #f7f9fc;
            --bg-white: #ffffff;
            --card-shadow: 0 4px 20px rgba(0, 0, 0, 0.06);
            --card-hover-shadow: 0 8px 30px rgba(0, 0, 0, 0.12);
            --border-radius: 12px;
            --transition: all 0.25s ease;
            --font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
        }

        /* ── Reset & Base ── */
        *,
        *::before,
        *::after {
            box-sizing: border-box;
            margin: 0;
            padding: 0;
        }
        html {
            scroll-behavior: smooth;
        }
        body {
            font-family: var(--font-family);
            background: var(--bg-light);
            color: var(--text-dark);
            line-height: 1.7;
            padding: 0;
            margin: 0;
        }
        a {
            text-decoration: none;
            color: var(--secondary);
        }
        a:hover {
            color: var(--secondary-hover);
            text-decoration: underline;
        }
        img {
            max-width: 100%;
            height: auto;
            display: block;
        }
        .container {
            max-width: 1200px;
            margin: 0 auto;
            padding: 0 24px;
        }
        .text-center {
            text-align: center;
        }

        /* ── Buttons ── */
        .btn {
            display: inline-block;
            padding: 1px 9px;
            border-radius: 30px;
            font-weight: 600;
            font-size: 1rem;
            cursor: pointer;
            transition: var(--transition);
            border: none;
            text-align: center;
            background: var(--secondary);
            color: #fff;
            text-shadow: 2px 2px 2px #000;   
            border: 2px solid #FFF;  
        }
        .btn:hover {
            background: var(--secondary-hover);
            text-decoration: none;
            transform: translateY(-2px);
            box-shadow: 0 8px 20px rgba(30, 136, 179, 0.3);
        }
        .btn-sm {
            padding: 8px 16px;
            font-size: 0.85rem;
        }
        .btn-outline {
            background: transparent;
            color: var(--secondary);
            border: 2px solid var(--secondary);
        }
        .btn-outline:hover {
            background: var(--secondary);
            color: #fff;
            border-color: var(--secondary);
        }

        /* ── Header ── */
        .site-header {
            background: var(--primary);
            color: #fff;
            padding: 16px 0;
            position: sticky;
            top: 0;
            z-index: 1000;
            box-shadow: 0 2px 12px rgba(0, 0, 0, 0.15);
        }
        .site-header .container {
            display: flex;
            align-items: center;
            justify-content: space-between;
            flex-wrap: wrap;
            gap: 12px;
        }
        .logo {
            font-weight: 800;
            font-size: 1.25rem;
            letter-spacing: -0.5px;
            display: flex;
            align-items: center;
            gap: 8px;
            color: #fff;
        }
        .logo span {
            color: var(--accent);
        }
        .logo small {
            font-weight: 400;
            font-size: 0.7rem;
            opacity: 0.7;
            margin-left: 6px;
        }
        .nav-links {
            display: flex;
            gap: 20px;
            font-size: 0.9rem;
            font-weight: 500;
            align-items: center;
            flex-wrap: wrap;
        }
        .nav-links a {
            color: #d0e4f0;
            transition: color 0.2s;
        }
        .nav-links a:hover {
            color: #fff;
            text-decoration: none;
        }
        .nav-links .btn-nav {
            background: var(--secondary);
            color: #fff;
            padding: 6px 16px;
            border-radius: 30px;
            font-weight: 600;
            transition: background 0.2s;
        }
        .nav-links .btn-nav:hover {
            background: var(--secondary-hover);
            text-decoration: none;
        }

        /* ── Page Header ── */
        .page-hero {
            background: linear-gradient(145deg, var(--primary) 0%, var(--primary-light) 100%);
            color: #fff;
            padding: 40px 0 32px;
            border-bottom: 4px solid var(--secondary);
        }
        .page-hero h1 {
            font-size: 2.2rem;
            font-weight: 800;
            letter-spacing: -0.5px;
            line-height: 1.2;
            margin-bottom: 8px;
        }
        .page-hero h1 span {
            color: var(--accent);
        }
        .page-hero p {
            font-size: 1.0rem;
            opacity: 0.9;
            max-width: 680px;
        }

        /* ── Calculator Card ── */
        .calc-card {
            background: var(--bg-white);
            border-radius: var(--border-radius);
            padding: 32px 36px;
            margin: 32px 0 40px;
            box-shadow: var(--card-shadow);
            border: 1px solid #e8edf3;
        }
        .calc-card h2 {
            font-size: 1.5rem;
            font-weight: 700;
            margin-bottom: 6px;
            color: var(--primary);
        }
        .calc-card .subtitle {
            color: #4a5a6a;
            margin-bottom: 24px;
            font-size: 0.95rem;
        }
        .calc-grid {
            display: grid;
            grid-template-columns: 1fr 1fr;
            gap: 20px 28px;
            margin-bottom: 24px;
        }
        .calc-group label {
            display: block;
            font-weight: 600;
            font-size: 0.9rem;
            color: var(--primary);
            margin-bottom: 4px;
        }
        .calc-group label .unit {
            font-weight: 400;
            color: #6a7a8a;
            font-size: 0.85rem;
        }
        .calc-group input,
        .calc-group select {
            width: 100%;
            padding: 10px 14px;
            border: 1px solid #cbd8e5;
            border-radius: 8px;
            font-size: 1rem;
            font-family: inherit;
            background: #fff;
            transition: border 0.2s;
        }
        .calc-group input:focus,
        .calc-group select:focus {
            outline: none;
            border-color: var(--secondary);
            box-shadow: 0 0 0 3px rgba(30, 136, 179, 0.15);
        }
        .calc-group .hint {
            font-size: 0.8rem;
            color: #6a7a8a;
            margin-top: 4px;
        }
        .calc-actions {
            display: flex;
            flex-wrap: wrap;
            gap: 12px;
            align-items: center;
            margin-top: 4px;
        }
        .calc-actions .btn {
            padding: 12px 32px;
        }
        .calc-actions .btn-secondary {
            background: #eef3f8;
            color: var(--text-dark);
        }
        .calc-actions .btn-secondary:hover {
            background: #d5e3ee;
            transform: none;
            box-shadow: none;
        }

        /* ── Results ── */
        .calc-results {
            background: #f0f6fc;
            border-radius: 10px;
            padding: 20px 24px;
            margin-top: 20px;
            border: 1px solid #d5e3ee;
            display: none;
        }
        .calc-results.show {
            display: block;
        }
        .calc-results .result-grid {
            display: grid;
            grid-template-columns: 1fr 1fr 1fr;
            gap: 16px;
        }
        .calc-results .result-item {
            background: #fff;
            border-radius: 8px;
            padding: 14px 18px;
            text-align: center;
            border: 1px solid #d5e3ee;
        }
        .calc-results .result-item .label {
            font-size: 0.8rem;
            color: #4a5a6a;
            font-weight: 500;
            text-transform: uppercase;
            letter-spacing: 0.3px;
        }
        .calc-results .result-item .value {
            font-size: 1.6rem;
            font-weight: 700;
            color: var(--primary);
            margin-top: 2px;
        }
        .calc-results .result-item .value .unit {
            font-size: 0.9rem;
            font-weight: 400;
            color: #4a5a6a;
        }
        .calc-results .result-item .note {
            font-size: 0.8rem;
            color: #4a5a6a;
            margin-top: 4px;
        }
        .calc-results .result-item.highlight {
            background: var(--primary);
            border-color: var(--primary);
        }
        .calc-results .result-item.highlight .label {
            color: rgba(255, 255, 255, 0.7);
        }
        .calc-results .result-item.highlight .value {
            color: #fff;
        }
        .calc-results .result-item.highlight .value .unit {
            color: rgba(255, 255, 255, 0.6);
        }
        .calc-results .result-item.highlight .note {
            color: rgba(255, 255, 255, 0.6);
        }

        @media (max-width: 768px) {
            .calc-grid {
                grid-template-columns: 1fr;
            }
            .calc-results .result-grid {
                grid-template-columns: 1fr;
            }
            .calc-card {
                padding: 20px 18px;
            }
            .page-hero h1 {
                font-size: 1.6rem;
            }
            .calc-results .result-item .value {
                font-size: 1.3rem;
            }
        }

        /* ── Content Sections ── */
        .content-section {
            margin: 40px 0;
        }
        .content-section h2 {
            font-size: 1.6rem;
            font-weight: 700;
            color: var(--primary);
            margin-bottom: 12px;
        }
        .content-section h3 {
            font-size: 1.2rem;
            font-weight: 600;
            color: var(--primary-light);
            margin: 24px 0 8px;
        }
        .content-section p {
            margin-bottom: 14px;
            color: #2a3a4a;
        }
        .content-section ul,
        .content-section ol {
            margin: 10px 0 16px 24px;
        }
        .content-section li {
            margin-bottom: 6px;
        }

        .formula-box {
            background: #f0f6fc;
            border-radius: 8px;
            padding: 18px 22px;
            margin: 16px 0;
            border-left: 4px solid var(--secondary);
            font-family: 'Courier New', monospace;
            font-size: 1.05rem;
            overflow-x: auto;
        }
        .formula-box .math {
            font-weight: 600;
            color: var(--primary);
        }

        .example-box {
            background: #f8fafc;
            border-radius: 8px;
            padding: 18px 22px;
            margin: 16px 0;
            border: 1px solid #e8edf3;
        }
        .example-box .step {
            margin: 6px 0;
            padding-left: 8px;
            border-left: 3px solid var(--secondary);
        }

        .benefits-grid {
            display: grid;
            grid-template-columns: 1fr 1fr;
            gap: 12px 24px;
            margin: 12px 0 16px;
        }
        .benefits-grid .item {
            display: flex;
            align-items: flex-start;
            gap: 10px;
        }
        .benefits-grid .item i {
            color: var(--secondary);
            font-size: 1.1rem;
            margin-top: 3px;
            flex-shrink: 0;
        }

        .related-grid {
            display: grid;
            grid-template-columns: 1fr 1fr;
            gap: 12px;
            margin: 12px 0;
        }
        .related-grid a {
            display: block;
            padding: 10px 16px;
            background: #f8fafc;
            border-radius: 8px;
            border: 1px solid #e8edf3;
            transition: var(--transition);
            font-weight: 500;
        }
        .related-grid a:hover {
            background: #eef3f8;
            border-color: var(--secondary);
            text-decoration: none;
            transform: translateY(-2px);
        }

        /* ── FAQ ── */
        .faq-item {
            border: 1px solid #e8edf3;
            border-radius: 8px;
            margin-bottom: 10px;
            overflow: hidden;
        }
        .faq-item summary {
            padding: 14px 20px;
            font-weight: 600;
            cursor: pointer;
            background: #f8fafc;
            transition: background 0.2s;
            list-style: none;
            display: flex;
            justify-content: space-between;
            align-items: center;
        }
        .faq-item summary::-webkit-details-marker {
            display: none;
        }
        .faq-item summary::after {
            content: "+";
            font-size: 1.3rem;
            font-weight: 300;
            color: var(--secondary);
        }
        .faq-item[open] summary::after {
            content: "−";
        }
        .faq-item summary:hover {
            background: #eef3f8;
        }
        .faq-item .faq-answer {
            padding: 0 20px 18px;
            color: #2a3a4a;
        }

        /* ── Footer ── */
        .site-footer {
            background: var(--primary);
            color: #b0c8d8;
            padding: 40px 0 28px;
            border-top: 4px solid var(--secondary);
            margin-top: 40px;
        }
        .site-footer .container {
            display: grid;
            grid-template-columns: 2fr 1fr 1fr;
            gap: 32px;
        }
        .site-footer h5 {
            color: #fff;
            font-size: 1rem;
            margin-bottom: 12px;
        }
        .site-footer a {
            color: #b0c8d8;
        }
        .site-footer a:hover {
            color: #fff;
            text-decoration: none;
        }
        .site-footer ul {
            list-style: none;
            padding: 0;
        }
        .site-footer ul li {
            padding: 4px 0;
        }
        .footer-bottom {
            grid-column: 1 / -1;
            border-top: 1px solid rgba(255, 255, 255, 0.08);
            padding-top: 20px;
            margin-top: 12px;
            font-size: 0.85rem;
            text-align: center;
        }
        @media (max-width: 768px) {
            .site-footer .container {
                grid-template-columns: 1fr;
                gap: 24px;
            }
            .benefits-grid {
                grid-template-columns: 1fr;
            }
            .related-grid {
                grid-template-columns: 1fr;
            }
        }

        /* ── Back to top ── */
        .back-to-top {
            position: fixed;
            bottom: 30px;
            right: 30px;
            background: var(--secondary);
            color: #fff;
            width: 48px;
            height: 48px;
            border-radius: 50%;
            display: flex;
            align-items: center;
            justify-content: center;
            font-size: 1.2rem;
            box-shadow: 0 4px 16px rgba(0, 0, 0, 0.2);
            transition: var(--transition);
            z-index: 999;
            border: none;
            cursor: pointer;
            opacity: 0;
            visibility: hidden;
            transform: translateY(20px);
        }
        .back-to-top.visible {
            opacity: 1;
            visibility: visible;
            transform: translateY(0);
        }
        .back-to-top:hover {
            background: var(--secondary-hover);
            transform: translateY(-4px);
        }

        /* ── Material quick reference ── */
        .material-ref {
            background: #f8fafc;
            border-radius: 8px;
            padding: 14px 18px;
            margin: 12px 0;
            border: 1px solid #e8edf3;
            font-size: 0.9rem;
        }
        .material-ref table {
            width: 100%;
            border-collapse: collapse;
            font-size: 0.9rem;
        }
        .material-ref th {
            text-align: left;
            font-weight: 600;
            padding: 6px 8px 6px 0;
            border-bottom: 1px solid #d5e3ee;
            color: var(--primary);
        }
        .material-ref td {
            padding: 6px 8px 6px 0;
            border-bottom: 1px solid #eef3f8;
        }
        .material-ref tr:last-child td {
            border-bottom: none;
        }