setInterval ( "changeAd()", 5000 );
var currentAd = 0;
var onAd = false;
function changeAd() {
  if (onAd) { return; }
  $('listingAd'+currentAd).setStyle('visibility', 'hidden');
  if ($('listingAd'+(currentAd+1))) {
    currentAd = currentAd+1;
  } else {
    currentAd = 0;
  }
  $('listingAd'+currentAd).setStyle('visibility', 'visible');
}