var sidebar = null;

var doScroll = function() {
	if(sidebar == null) sidebar = document.getElementById("sidebar");
	var top = document.body.scrollTop;
	if(top == 0 && typeof(window.pageYOffset) != "undefined") {
		top = window.pageYOffset;
	} else
	if(top == 0 && typeof(document.documentElement) != "undefined") {
		top = document.documentElement.scrollTop;
	}
	
	var height = document.documentElement.scrollHeight;
	var d = height - top;

	if(d > 800) {
		sidebar.style.marginTop = Math.max(0, top - 300) + "px";
	}
};

if(document.addEventListener) {
	document.addEventListener("scroll", doScroll, false);
} else {
	window.onscroll = doScroll;
}