Footer Stick

#header {
    background: Green;
    height: 40px;
    }

#content {
          float: right;
          width: 80%;
          background: Olive;
        }

#sidebar {
      float: left;
      width: 20%;
      background: Gray;
      }

#footer {
  height: 50px;
  background: Background;
  width:770px;
  margin: -50px auto 0;
  }