.back-to-top {
  box-sizing: border-box;
  position: fixed;
  bottom: -100px;
  right: 50px;
  z-index: 1050;
  padding: 0 6px;
  background: #222;
  font-size: 12px;
  opacity: 0.6;
  color: #fff;
  cursor: pointer;
  text-align: center;
  -webkit-transform: translateZ(0);
  transition-property: bottom;
  transition-duration: 0.5s;
  transition-timing-function: ease-in-out;
  transition-delay: 0s;
  height: 40px;
  width:40px;

}
@media (max-width: 767px) {
  .back-to-top {
    display: none;
  }
}
@media (min-width: 768px) and (max-width: 991px) {
  .back-to-top {
    display: none;
  }
}
.back-to-top.back-to-top-on {
  bottom: 40px;
}