body {
    font-family: sans-serif;
    background: #f5f5f5;
    padding: 2em;
    text-align: center;
}
.container {
    background: white;
    display: inline-block;
    padding: 2em;
    border-radius: 10px;
    box-shadow: 0 0 10px rgba(0,0,0,0.1);
}
input[type="number"] {
    width: 60px;
}
.result {
    margin-top: 1em;
    font-size: 1.2em;
    background: #eee;
    padding: 1em;
    border-radius: 8px;
}