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var stickyNavTop = $('.nav').offset().top;  
  
var stickyNav = function(){  
  
  var scrollTop = $(window).scrollTop();  
         
  if (scrollTop > stickyNavTop) {   
      $('.nav').addClass('sticky');  
  } else {  
      $('.nav').removeClass('sticky');   
  }  
  
};

$(document).ready(function() { 
  stickyNav();
});  

$(window).scroll(function() {  
  stickyNav();  
});

.sticky {  
  position: fixed;   
  left: 0;  
  top: 0;  
  width: 100%; 
  z-index: 100; 
}