* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: Arial, sans-serif;
}

body {
    background: #f4f4f4;
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
}

.calculator {
    background: #222;
    border-radius: 10px;
    box-shadow: 0 4px 10px rgba(0, 0, 0, 0.2);
    max-width: 600px;
    width: 90%;
    padding: 15px;
}

#display {
    width: 100%;
    height: 50px;
    background: #fff;
    border: none;
    border-radius: 5px;
    color: #333;
    font-size: 22px;
    text-align: right;
    padding: 10px;
    margin-bottom: 10px;
}

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

button {
    width: 100%;
    height: 60px;
    font-size: 20px;
    background: #444;
    color: #fff;
    border: none;
    border-radius: 5px;
    cursor: pointer;
    transition: background 0.2s;
}

button:hover {
    background: #666;
}

.equal {
    background: #f57c00;
}

.equal:hover {
    background: #ff9800;
}