text: var slideIndex = 1;
  showDivs(slideIndex);
  
  function plusDivs(n) {
      showDivs(slideIndex += n);
  }
  
  function showDivs(n) {
      var i;
      var x = document.getElementsByClassName("zaal");
      if (n > x.length) {slideIndex = 1} 
      if (n < 1) {slideIndex = x.length} ;
      for (i = 0; i < x.length; i++) {
          x[i].style.display = "none"; 
      }
      x[slideIndex-1].style.display = "block"; 
  }