var timeout = 1000*8;
var current = 1;
function change(cnt) {
    if(current > 0)
        document.getElementById('ad' + current).style.display = 'none';
    current++;
    if(current > cnt)
        current = 1;
    document.getElementById('ad' + current).style.display = 'inline';
    window.setTimeout("change(" + cnt + ")", timeout);
}