function cleanInput(obj,e) {
	var title = obj.getAttribute("title");
	var valor = obj.value;
	
	if(valor == title && e == "focus") obj.value = "";
	if(valor == "" && e == "blur") obj.value = title;
}

function valor () {
    var er = /[^0-9-]/;
    if(er.test (document.form_comprar.qtn.value)){
        document.form_comprar.qtn.value = document.form_comprar.qtn.value.slice(0, (document.form_comprar.qtn.value.length) - 1);
    }
}