In mathematics, the factorial of integer 'n' is written as 'n!'.
It is equal to the product of n and every integer preceding it.
For example: **5! = 1 x 2 x 3 x 4 x 5 = 120**
Your mission is simple: write a function that takes an integer 'n' and returns 'n!'.
You are guaranteed an integer argument. For any values outside the positive range, return `null`, `nil` or `None` .
**Note:** 0! is always equal to 1. Negative values should return null; 
For more on Factorials : http://en.wikipedia.org/wiki/Factorial
import math

def factorial(n):
    if n < 0:
        return None
    return math.factorial(n)
import math

def factorial(n):
    if n < 0:
        return None
    return math.factorial(n)