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

body {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    font-family: Arial, sans-serif;
    background-color: #f3f4f6;
}

.calculator {
    width: 100%;
    max-width: 320px;
    background-color: #222;
    padding: 20px;
    border-radius: 10px;
    box-shadow: 0 4px 10px rgba(0, 0, 0, 0.1);
}

.calculator-screen {
    width: 100%;
    height: 50px;
    background-color: #333;
    color: #fff;
    font-size: 24px;
    text-align: right;
    padding: 10px;
    border: none;
    border-radius: 5px;
    margin-bottom: 10px;
}

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

.button {
    width: 100%;
    padding: 20px;
    font-size: 18px;
    color: #fff;
    background-color: #444;
    border: none;
    border-radius: 5px;
    cursor: pointer;
    transition: background-color 0.3s;
}

.button:hover {
    background-color: #555;
}

.operator {
    background-color: #f57c00;
}

.operator:hover {
    background-color: #fb8c00;
}

.equal-sign {
    background-color: #4caf50;
    grid-column: span 2;
}

.equal-sign:hover {
    background-color: #66bb6a;
}