Python, <s>43</s> 38
=

<!-- language: lang-python -->

    import math
    f=lambda n:math.gamma(n+1)

**Explanation:** The [gamma function][1] is a very quickly-growing complex function which, at integer values, is equal to the factorial of one less than the number. So we add one to n and take the gamma function of it.

I hope this isn't considered cheating, since the gamma function is not technically able to directly calculate the factorial.
 

  [1]: https://en.wikipedia.org/wiki/Gamma_function