Restrict multiple select list to max of 2 selections

gistfile1.txt

		<script>
			jQuery(document).ready(function($){
				$('#subjects option').click(function() 
    {
        var items = $(this).parent().val();
        if (items.length > 2) {
                       alert("You can only select 2 values at a time");
           $('#subjects option:selected').prop("selected",false);
        }
    });
			});
		</script>