body{
  margin: 0;
  padding: 0;
}
.cont{
  text-align: center;
  margin-top: 360px;
}
.btn{
  border: 1px solid green;
  background: none;
  padding: 10px 20px;
  font-size: 20px;
  cursor: pointer;
  margin: 10px;
  transition: 0.8s;
  position: relative;
  overflow: hidden;
}
.btn2{
  color: green;
}
.btn2:hover{
  color: #fff;
}
.btn::before{
  content: "";
  position: absolute;
  left: 0;
  width: 100%;
  height: 0%;
  background: green;
  z-index: -1;
  transition: 0.8s;
}
.btn2::before{
  bottom: 0;
  border-radius: 50% 50% 0 0;
}
.btn2:hover::before{
  height: 180%;
}