function verify() {
var themessage = "Folgende Felder müssen korrekt ausgefüllt werden: ";
if (document.form.Nachname.value=="") {
themessage = themessage + " - Nachname";
}
if (document.form.Vorname.value=="") {
themessage = themessage + " -  Vorname";
}
if (document.form.Strasse.value=="") {
themessage = themessage + " -  Strasse";
}
if (document.form.Nationalitaet.value=="") {
themessage = themessage + " -  Nationalitaet";
}
if (document.form.PLZ.value=="") {
themessage = themessage + " -  PLZ";
}
if (document.form.Ort.value=="") {
themessage = themessage + " -  Ort";
}
if (document.form.email.value=="") {
themessage = themessage + " -  email";
}
if (document.form.Anreisedatum.value=="") {
themessage = themessage + " -  Anreisedatum";
}
if (document.form.Abreisedatum.value=="") {
themessage = themessage + " -  Abreisedatum";
}
if (document.form.betrag.value=="") {
themessage = themessage + " -  Anzahlungsbetrag";
}
//alert if fields are empty and cancel form submit
if (themessage == "You are required to complete the following fields: ") {
document.form.submit();
}
else {
alert(themessage);
return false;
   }
}