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

body {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    background: #1d1d1d;
}

.calculator {
    width: 340px;
    background: #2b2b2b;
    border-radius: 20px;
    padding: 20px;
    box-shadow: 0 10px 25px rgba(0, 0, 0, .4);
}

.display {
    height: 90px;
    background: #111;
    border-radius: 15px;
    display: flex;
    justify-content: flex-end;
    align-items: center;
    padding: 20px;
    margin-bottom: 20px;
}

.display span {
    color: #fff;
    font-size: 42px;
    font-weight: bold;
}

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

button {
    height: 65px;
    border: none;
    border-radius: 15px;
    background: #3d3d3d;
    color: #fff;
    font-size: 24px;
    cursor: pointer;
    transition: .2s;
}

button:hover {
    background: #555;
}

button:active {
    transform: scale(.95);
}

button:nth-child(4),
button:nth-child(8),
button:nth-child(12),
button:nth-child(16),
button:last-child {
    background: #ff9500;
}

button:nth-child(4):hover,
button:nth-child(8):hover,
button:nth-child(12):hover,
button:nth-child(16):hover,
button:last-child:hover {
    background: #ffad33;
}

.zero {
    grid-column: span 2;
}