* {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }

        body {
            font-family: 'Arial', sans-serif;
            background: linear-gradient(135deg, #a1c4fd 0%, #c2e9fb 50%, #f0faff 100%);
            display: flex;
            justify-content: center;
            align-items: center;
            min-height: 100vh;
            padding: 20px;
        }

        .calculator {
            background: rgba(45, 45, 45, 0.1);
            /*semi-transparent */
            backdrop-filter: blur(15px);
            /* frosted glass effect*/
            border-radius: 25px;
            padding: 30px;
            max-width: 350px;
            width: 100%;
            border: 2px solid rgba(255, 255, 255, 0.25);
            box-shadow: 0 10px 30px rgba(0, 0, 0, 0.2), inset 0 -2px 10px rgba(255, 255, 255, 0.1);

        }

        .display {
            background: #1a1a1a;
            border: 2px solid #444;
            border-radius: 20px;
            font-size: 2.5rem;
            font-weight: 300;
            font-family: 'Courier New', monospace;
            text-align: right;
            padding: 25px;
            margin-bottom: 25px;
            color: #00ff88;
            box-shadow: inset 0 4px 15px rgba(0, 0, 0, 0.3);
            min-height: 90px;
            display: flex;
            align-items: center;
            justify-content: flex-end;
            overflow: hidden;
        }

        .buttons {
            display: grid;
            grid-template-columns: repeat(4, 1fr);
            gap: 15px;
        }

        .btn {
            border: none;
            border-radius: 15px;
            font-size: 1.2rem;
            font-weight: 600;
            padding: 20px;
            cursor: pointer;
            transition: all 0.3s cubic-bezier(.25, .8, .25, 1);
            box-shadow: 0 4px 15px rgba(0, 0, 0, 0.1);
        }

        .btn:hover {
            transform: translateY(-2px);
            box-shadow: 0 6px 20px rgba(0, 0, 0, 0.15);
            transform: translateY(-2px) scale(1.05);
        }

        .btn:active {
            transform: translateY(0);
            box-shadow: 0 2px 10px rgba(0, 0, 0, 0.2);
        }

        .btn-number {
            background: #f8f9fa;
            color: #333;
            box-shadow: 0 4px 15px rgba(0, 0, 0, 0.15), 0 2px 8px rgba(0, 0, 0, 0.1);
        }

        .btn-number:hover {
            background: #e9ecef;
            box-shadow: 0 6px 20px rgba(0, 0, 0, 0.2), 0 3px 10px rgba(0, 0, 0, 0.15);
        }

        .btn-operator {
            background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
            color: white;
        }

        .btn-operator:hover {
            background: linear-gradient(135deg, #5a6fd8 0%, #6a4190 100%);
        }

        .btn-clear {
            background: linear-gradient(135deg, #ff6b6b 0%, #ee5a52 100%);
            color: white;
        }

        .btn-clear:hover {
            background: linear-gradient(135deg, #ff5252 0%, #d32f2f 100%);
        }

        .btn-equals {
            background: linear-gradient(135deg, #51cf66 0%, #40c057 100%);
            color: white;
        }

        .btn-equals:hover {
            background: linear-gradient(135deg, #37b24d 0%, #2f9e44 100%);
        }

        .btn-zero {
            grid-column: span 1;
        }

        .btn-equals-wide {
            grid-column: span 2;
        }

        .equals-tall {
            grid-row: span 2;
        }

        @media (max-width: 400px) {
            .calculator {
                padding: 8px;
                margin: 8px;
            }

            .display {
                font-size: 1.5rem;
                padding: 15px;
            }

            .btn {
                padding: 15px;
                font-size: 1rem;
                 gap: 10px;
            }
        }