$(function(){
             $('nav').css({top:'0'})
          
            $(window).trigger('scroll')
                

            var offset = 150;
            $(window).scroll(function(){
                var st = $(this).scrollTop();
            if (st > offset) {
                $('nav').css({position:'fixed',top:0});
            }else{
                $('nav').css({position:'absolute',top:'0'})
            }

                
            });
});