function rolll(){ //页面打开后缓冲多长时间执行，单位毫秒
	window.setTimeout(itemRolll,100);
}

function itemRolll(){
	var _topp=document.documentElement.scrollTop;
	var _objj=document.getElementById("roll_l");
	var _timess;
	
	if(!_objj.style.top){
		_objj.style.top=80+_topp+"px"; //
		_timess=500;
	}else{
		var obj_topp=parseInt(_objj.style.top)-90; //根据右侧位置，确定基准值
		_timess=100;
		if(obj_topp!=_topp){
			var yOffsett = Math.ceil(Math.abs(obj_topp - _topp) / 20);
			if(obj_topp>_topp){
				yOffsett=-yOffsett;
			}
			_objj.style.top=parseInt(_objj.style.top,10)+yOffsett+"px";
			_timess=10
		}
	}
	window.setTimeout(itemRolll,_timess);
}