Jquery get selected text value

$(document).ready(function() {
		$(".contenttext").mouseup(function(e) {
			var txt;
			var parentOffset = $(this).offset();
			var x = e.pageX - parentOffset.left;
			var y = e.pageY - parentOffset.top;
			txt = window.getSelection();
			if (txt.toString().length > 1) {
				alert(txt);
			}
		});
	});