I tried again too, and it works indeed... First time, I didn't remeber that in C cast was implicit in an operation like that, maybe the parentheses I've add (somthing like (int)'f'), or a typo I didn't saw. Anyway, that's my bad, sorry!

That's what I tryed first but he told me that I was using a forbiden caracter!? Did they revoke the modulo char? If that's it, they should put it in the forbiden caracters list...