var lastScrollY = 0; 
function rollimg(){ 
	var diffY = document.body.scrollTop;
	if (diffY <= 0) {
		diffY = document.documentElement.scrollTop;
	}
	var percent =.1*(diffY-lastScrollY);
	if ( percent >0 ) {
		percent=Math.ceil(percent);
	} else {
		percent=Math.floor(percent);
	}
	var top = parseInt(document.getElementById("magMainAd").style.top);
	document.getElementById("magMainAd").style.top = (top += percent) + "px";
	lastScrollY += percent; 
}

window.setInterval("rollimg()",1);