Cannot calculate factorials bigger than 20! ! How to do so?

Question

I am using unsigned long long integer format in order to calculate big factorials. However my code fails at some point can you have a look at it? Actually it is part of a la

Answer 1

The limit on an unsigned long long is 18446744073709551615, or about 1.8e+19. 20! is about 2.4e+18, so within range, however 21! is about 5.1e+19, exceeding the maximum size of an unsigned long long.

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