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

Range of unsigned long long is usually 0 to 2^64 - 1 (18,446,744,073,709,551,615). 21! goes out of this range.