$(function() {
  upDown();
});
$(window).scroll(function() {
  var winST = $(window).scrollTop();
  var winH = $(window).height();
  var doH = $(document).height();
  if (winST > 0) {
    $(".back-to-top").removeClass("disable");
    if (winST == doH - winH && $(".back-to-bottom").css("display") == "block") {
      $(".back-to-bottom").addClass("disable");
    } else {
      $(".back-to-bottom").removeClass("disable");
    }
  } else {
    $(".back-to-top").addClass("disable");
  }
});
function upDown() {
  var winST = $(window).scrollTop();
  if (winST === 0) {
    $(".back-to-top").addClass("disable");
  }
  $(".topbtn a").click(function(e) {
    var winH = $(window).height();
    var doH = $(document).height();
    var speed = 600;
    if (!$(this).hasClass("back-to-top")) {
      $("html,body").stop().animate({
        scrollTop: doH - winH
      }, speed);
    } else {
      $("html,body").stop().animate({
        scrollTop: 0
      }, speed);
    }
    e.preventDefault();
  });
}