function banner(img_source,url,alt,chance) {
   this.img_source = img_source;
   this.url = url;
   this.alt = alt;
   this.chance = chance;
}
function display() {
   with (this) document.write("<a href=" + url + " rel='nofollow'><img src='" + img_source + "' width='494' height='120' border='0' alt='" + alt + "'></a>");
}
banner.prototype.display = display;
banners = new Array();
banners[0] = new banner("http://i30.tinypic.com/2zjanev.jpg",
                        "http://www.demotywery.pl target='_blank'",
                        "Demotywujace demotywatory",
                        10);
banners[1] = new banner("http://i29.tinypic.com/312zq88.jpg",
                      "http://www.motywery.pl target='_blank'",
                        "Motywujace motywatory",
                        20);
banners[2] = new banner("http://i30.tinypic.com/2zjanev.jpg",
                        "http://www.demotywery.pl target='_blank'",
                        "Demotywujace demotywatory",
                        10);
banners[3] = new banner("http://i29.tinypic.com/312zq88.jpg",
                        "http://www.motywery.pl target='_blank'",
                        "Motywujace motywatory",
                        20);
sum_of_all_chances = 0;
for (i = 0; i < banners.length; i++) {
   sum_of_all_chances += banners[i].chance;
}
function display_ban() {
   chance_limit = 0;
   randomly_selected_chance = Math.round((sum_of_all_chances - 1) * Math.random()) + 1;
   for (i = 0; i < banners.length; i++) {
      chance_limit += banners[i].chance;
      if (randomly_selected_chance <= chance_limit) {
         document.write("<a href=" + banners[i].url + " rel='nofollow'><img src='" + banners[i].img_source + "' width='494' height='120' border='0' alt='" + banners[i].alt + "'></A>");
         return banners[i];
         break;
      }
   }
}