var rand1 = Math.random();
var rn1 = Math.floor(rand1*100+1);
 
<!-- TD Server 120x600
if(rn1>=0 && rn1<=100) 
{
var uri = 'http://impes.tradedoubler.com/imp?type(js)pool(145553)a(914690)' + new String (Math.random()).substring (2, 11);
document.write('<sc'+'ript type="text/javascript" src="'+uri+'" charset="ISO-8859-1"></sc'+'ript>');
}