Build custom validators

Hi, I'm looking for a calculation solution for the event registration form.
I have a ticket list field "tickets" (Anzahl Tickets) and two additional list field called "vegi" (Anzahl Vegi) and "fleisch" (Anzahl Fleisch). If I enter e.g. the numer 3 into the Ticket field the customer need to enter the equal amount of 3 in total.... in the below sample 2 and 1. Everythning else gives an alert message.

The solution I'm looking for, that the customer need to choose exactly the value from the ticket field  

I know with this great piece of software it's possible:  Validate Field Data - Server Side | Events Booking Documentation
But I really appriciate if someone can give me any support.

Thanks
Bjoern