window.onload = function() {
	var backtop = document.getElementById("backtop");
	var timer = null;
	var isTop = true;
	var clientHeight = document.documentElement.clientHeight;

	window.onscroll = function(){
		var osTop = document.documentElement.scrollTop || document.body.scrollTop;
		if(osTop >= clientHeight) {
			backtop.style.display = "block";
		} else {
			backtop.style.display = "none";
		}
		if(!isTop) {
			clearInterval(timer);
		}
		isTop = false;
	};

	backtop.onclick = function() {
		timer = setInterval(function(){
			var osTop = document.documentElement.scrollTop || document.body.scrollTop;
			var ispeed = Math.floor(-osTop / 6);
			document.documentElement.scrollTop = document.body.scrollTop = osTop + ispeed;

			isTop = true;
			if(osTop == 0) 
				clearInterval(timer);
		}, 30);
	};
};