var returnValue = true, 
imgs=["../../images/spanish/top_science.jpg", "../../images/spanish/top_science2.jpg"]; len=(imgs.length);
if ((len*2)%2==1){returnValue=false;}

function showAd(){
if(returnValue==false)
	{document.write('<img src="Blank.gif" alt="" >');return false;}
	var rand=Math.floor(len*Math.random());
	document.write("<img src=\""+imgs[rand]+"\" usemap='#nav_map' border=0>");
}