var page, numPages;

function show(id) {
  document.getElementById(id).style.display = 'block';
}

function hide(id) {
  document.getElementById(id).style.display = 'none';
}

window.onload = function() {
  page = window.location.hash.replace(/#/, '');
  if (!page) page = 1;
  change();
}

function next() {
  hide('page' + page++);
  change();
}

function prev() {
  hide('page' + page--);
  change();
}

function change() {
  show('page' + page);
  if (page == 1) hide('prev'); else show('prev');
  if (page == numPages) hide('next'); else show('next');
  window.location.hash = page;
}