* {
    box-sizing: border-box;
    font-family: Arial, sans-serif;
}

body {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    background: #eee;
    margin: 0;
}

.calculator {
    background: #222;
    padding: 20px;
    border-radius: 16px;
    box-shadow: 0 0 20px #000;
    width: 380px;
}

.display {
    background: black;
    color: #0f0;
    padding: 20px;
    font-size: 2em;
    text-align: right;
    border-radius: 8px;
    margin-bottom: 20px;
    min-height: 50px;
    overflow-x: auto;
}

.buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 10px;
}

.btn {
    padding: 20px;
    font-size: 1.2em;
    border: none;
    border-radius: 8px;
    background: #333;
    color: white;
    cursor: pointer;
    transition: 0.2s;
}

.btn:hover {
    background: #444;
}

.btn.operator {
    background: #f57c00;
}

.btn.operator:hover {
    background: #fb8c00;
}

.btn.equal {
    grid-row: span 2;
    background: #4caf50;
}

.btn.equal:hover {
    background: #66bb6a;
}

.btn.zero {
    grid-column: span 2;
}