enable/disable button with condition

Hello,
I have a form with several drop-down lists. The value of each list is added to the widget calculator.
I would like that the button "envoyer la demande"to be disabled if the amount is greater than 42.
Thanks
​

Salut DesCamps,
​
The best way to do this is probably with a Custom JavaScript function that is called when any of the drop-downs change.
​
Bob