$(function(){
  //Get our elements for faster access and set overlay width
  var div = $('#city_block'),
               ul = $('#city_list'),
               // unordered list's left margin
               ulPadding = 0;

  //Get menu width
  var divHeight = div.height();

  //Remove scrollbars
  div.css({overflow: 'hidden'});

  //Find last image container
  var lastLi = ul.find('li:last-child');

  //When user move mouse over menu
  div.mousemove(function(e){

    //As images are loaded ul height increases,
    //so we recalculate it each time
    var ulHeight = lastLi[0].offsetTop + lastLi.outerHeight() + ulPadding;

    var top = (e.pageY - div.offset().top) * (ulHeight-divHeight) / divHeight;
    div.scrollTop(top);
  });
});