Change some classes while scrolling

gistfile1.js

$(function() {
	$(window).scroll(function() {
		var scrolled = $(window).scrollTop();
		
		if (scrolled > 20) {
			$('header').addClass('topFixed');
		}
			
		if (scrolled > 50) {			
			$('header').addClass('small');
			
		} else {
			$('header').removeClass('small');
			$('header').removeClass('topFixed');
		}
	});
});