Navigates forward n pages
If n is negative, we will navigate in reverse
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();
}