.container {
    position: absolute;
    top: 50%;
    transform: translateY(-50%);
    width: 320px;
    box-shadow: 0 2px 10px rgba(0,0,0,0.3);
}

.calculator{
    max-width:320px;
}
.buttons{
    display:grid;
    grid-template-columns: repeat(4,1fr);
    border:0;
}
#result {
    font-family: 'Monospaced', monospace ;
    background: black;
    color: white;
    font-size: 2em;
    border:0;
    padding: 0.3em;
    text-align: right;
    width: 100%;
    height: 100px;
}

.btn {
    font-size:1em;
    height: 65px;
    border:1px solid black;
    margin: 1px;
}

.btn:hover{
    background: #fff;
}

.span-2 {
    grid-column: span 2;
}
.span-3 {
    grid-column: span 3;
}