window.onload = function(){
	var imgtop = document.getElementById('gottojpg');
	imgtop.onclick = function(){
		animate();
	}
	window.onscroll = function(){
		var top = document.documentElement.scrollTop;
		if(top>=100){
			imgtop.style.display = 'block';
		}
		if(top<100) imgtop.style.display = 'none';
	}
}

function animate(){
	var top = document.documentElement.scrollTop;
	var end = 0;
	var dur = 500;
	var t = 0;
	var ts = setInterval(function(){
		if(t>=dur) clearTimeout(ts);
		tt = Math.ceil(speed(t,top,-(top),dur));
		document.documentElement.scrollTop = tt
		t+=50;
	},50);
	function speed(t,b,c,d){
		if ((t/=d/2) < 1) return c/2*t*t*t + b;
		return c/2*((t-=2)*t*t + 2) + b;
	}
}