#back-to-top {
    position: fixed;
    bottom: 40px;
    right: 40px;
    z-index: 9999;
    width: 40px;
    height: 40px;
    text-align: center;
    line-height: 40px;
    background: #000;
    color: #fff;
    cursor: pointer;
    border: 0;
    border-radius: 2px;
    text-decoration: none;
    transition: all 0.5s ease;
    opacity: 1;
 
}

 

#back-to-top:hover {
    background: red;
}

#back-to-top.show {
    opacity: 0;
    bottom: -999px;
    transition: all 1s ease;
}