.bck {
  position: fixed;
  z-index: 999;
  bottom: 1rem;
  right: 1rem;
  background: #0000ab;
  color: #fff;
  visibility: hidden;
  width: 45px;
  height: 45px;
  border-radius: 50%;
  line-height: 45px;
  text-align: center;
  transition-property: all;
}

.bck.bck-on {
  visibility: visible;
}

.fade {
  opacity: 0;
}

.bck-on.fade {
  opacity: 1;
}

.rightToLeft {
  right: -50px;
}

.bck-on.rightToLeft {
  right: 1rem;
}

.bottomToTop {
  bottom: -50px;
}

.bck-on.bottomToTop {
  bottom: 1rem;
}

.bck-leftToRight {
  left: -50px;
}

.bck-on.leftToRight {
  left: 1rem;
}

.topToBottom {
  top: -50px;
}

.bck-on.topToBottom {
  top: 1rem;
}

@media screen and (max-width: 540px) {
  .bck {
    width: 40px;
    height: 40px;
    font-size: 13px;
    line-height: 40px;
  }
}