String.prototype.toInteger = function() {
 return this.replace(/[^0-9]/g, '') * 1;
}

function follow_banner(div_id, origin_top, limit_top, margin_top)
{
 var d, t, s, g, target_height;
 origin_top *= 1;
 limit_top *= 1;
 margin_top *= 1;
 
 d = document.getElementById(div_id);
 s = document.body.scrollTop + "";

 target_height = s.toInteger() + margin_top;
 if(target_height < limit_top) {
  target_height = origin_top;
 }
 
 t = d.style.top.toInteger();
 
 if(t != target_height) {
  g = Math.ceil((t - target_height) / 5);
  if(g > 50) {
   g = 50;
  } else if (g < -50) {
   g = -50;
  }
  d.style.top = (t - g) + "px";
 }
 
 setTimeout("follow_banner('" + div_id + "', " + origin_top + ", " + limit_top + ", " + margin_top + ");", 1);
}