//And manually call update once or at initial loading
//E.g. in jquery: $(document).on("load", update);
update();

setInterval(update, 2500);

function update()
{
      for (var i = 0; i < slides.length; i++) {
        slides[i].style.opacity = 0;

      }
      slides[current].style.opacity = 1;
      current = (current != slides.length - 1) ? current + 1 : 0;
}