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<script>
$(document).ready(function(){
var shrinkHeader = 300;
  $(window).scroll(function() {
    var scroll = getCurrentScroll();
      if ( scroll >= shrinkHeader ) {
           $('#menu').addClass('shrink');
        }
        else {
            $('#menu').removeClass('shrink');
        }
  });
function getCurrentScroll() {
    return window.pageYOffset || document.documentElement.scrollTop;
    }
});
</script>