 scrollStep = top/20 + 1;
    top -= scrollStep;
    if (top < 0)
      top = 0;
    document.body.scrollTop = top;
    requestAnimationFrame(funScroll);
  };
  funScroll();
}

window.addEventListener('scroll', function(e) {
  var backToTop = document.getElementById('back_to_top')
  if (document.body.scrollTop > 0) {
    backToTop.style.display = 'block';
  } else {	backToTop.style.display = 'none'  }});

//-->
</script>

<style type="text/css">
#back_to_top {
  bottom:20px; right:20px;
  width:30px; height