.back-to-top {

position: fixed;
left: 50%;
margin-left: 580px;
bottom: 37px;
background-color: #f8f8fd;
visibility: hidden;
opacity: 0;
transition: 0.4s ease;
box-shadow: 0 1px 3px 1px rgba(0, 0, 0, 0.3), 0 0 30px rgba(10, 10, 0, 0.1) inset;
a {
    display: block;
    background-color: #eee;
    width: 38px;
    height: 38px;
    line-height: 38px;
    text-align: center;
    font-size: 20px;
    color: #b4b4b4;
}

} .back-to-top-show{

visibility: visible;
opacity: 0.7;
&:hover{
    opacity: 1
}

} @media screen and (max-width:1200px) and (min-width: 770px) {

.back-to-top {
    left: 50%;
    margin-left: 45.5%;
}

} @media screen and (max-width: 770px) {

.back-to-top {
    right:2.5%;
    left: auto;
    // left: 50%;
    margin-left: 0px;
    a {
        width: 33px;
        height: 33px;
        line-height: 33px;
        text-align: center;
        font-size: 18px;
    }
}

}