code: function navigate(n) {
    var position = currentPosition();
    var numSlides = document.getElementsByClassName('slide').length;
  
    /* Positions are 1-indexed, so we need to add and subtract 1 */
    var nextPosition = (position - 1 + n) % numSlides + 1;
  
    /* Normalize nextPosition in-case of a negative modulo result */
    nextPosition = (nextPosition - 1 + numSlides) % numSlides + 1;
  
    document.getElementById('slide-' + position).classList.add('hidden');
    document.getElementById('slide-' + nextPosition).classList.remove('hidden');
  
    updateProgress();
    updateURL();
    updateTabIndex();
  }
docs: Navigates forward n pages
  If n is negative, we will navigate in reverse