backTop=function (btnId){
	var btn=document.getElementById(btnId);
	var d=document.documentElement;
	window.onscroll=set;
	btn.onclick=function (){
		btn.style.display="none";
		window.onscroll=null;
		this.timer=setInterval(function(){
			d.scrollTop-=Math.ceil(d.scrollTop*0.1);
			if(d.scrollTop==0) clearInterval(btn.timer,window.onscroll=set);
		},10);
	};
	function set(){btn.style.display=d.scrollTop?'block':"none"}
};
backTop('gotopbtn');