.gotop{
    position: fixed;
    right: 50px;
    bottom: 15vh;
    width: 50px;
    height: 50px;
    background: url(https://images.cnblogs.com/cnblogs_com/hhuiii/1571970/t_200426083304gotop.png) no-repeat;
    background-size:100% 100%;
    border-radius: 50%;
    text-align: center;
    line-height: 50px;
}