It seems that an overflow occurs before the test... I lowered n and p in the random tests. Could you try again? Hopefully it will work. It would be better if we had a BigInteger library at Codewars! I upvote your last post:-)

I understand now. For diagonal(183, 19) solutions return 401227812924226601. All solutions of random tests are less than ULLONG_MAX - 1000.

I am very sorry but maybe you should look again at your code. The return is an unsigned long long not a float number. It is not an issue of the kata but of your code.