jQuery - Action when user has scrolled

gistfile1.js

$(document).scroll(function(e) {
    var nav = $('nav'),
        page = $('body'),
        scrollValue = page[0].offsetTop - window.pageYOffset;
		
		if(scrollValue < -20 ) {
			nav.addClass('scrolled');
		}
		else {
			nav.removeClass('scrolled');
		}	
});