function getParameterByName(name) {
	name = name.replace(/[\[]/, "\[").replace(/[\]]/, "\]");
	var regex = new RegExp("[\?&]" + name + "=([^&#]*)"),
	results = regex.exec(location.search);
	return results === null ? "" : decodeURIComponent(results[1].replace(/\+/g, " "));
}

var getURL = getParameterByName('get');
var getIMG = getParameterByName('img');
var getKEY  = getParameterByName('key');
var getKEY2  = getParameterByName('key2');
if (getURL == "") {alert('Vuelve a la página anterior');}