#loading {
    position: fixed;
    top: 50%;
    transform:translateY(-50%);
    left: 46%;
}

#loading span {
    position: absolute;
    width: 20px;
    height: 20px;
    background: #3498db;
    opacity: 0.5;
    border-radius: 50%;
    animation: loading 1s infinite ease-in-out;
}

@keyframes loading {
    0% {
        opacity: 0.3;
        transform: translateY(0px);
    }
    50% {
        opacity: 1;
        transform: translateY(-20px);
        background: green;
    }
    100% {
        opacity: 0.3;
        transform: translateY(0px);
    }
}

@for $i from 1 to 5 {
    #loading span:nth-child(#{$i}) {
        left: 40 * ($i - 1) + px;
        /* animation-delay: 20 * ($i - 1) / 100 + s; */
        animation-delay: unquote($string: "0.") + ($i - 1) * 2 + s;
    }
}