var adcount = 2;
var currentadnumber = 0;
var adlist = new Array("ad1wrapper", "ad2wrapper");

function changead() {

    var currentad = document.getElementById(adlist[currentadnumber]);
    if (currentad != null) {
        currentad.style.display = "none";
        currentadnumber++;
        if (currentadnumber >= adcount) {
            currentadnumber = 0;
        }

        currentad = document.getElementById(adlist[currentadnumber]);
        if (currentad != null) {
            currentad.style.display = "block";
        }
    }

    setTimeout(changead, 11000);
}