#Factorial

In mathematics, the factorial of a positive integer n, denoted by n!, is the product of all positive integers less than or equal to n:

    n ! = n × ( n − 1 ) × ( n − 2 ) × ( n − 3 ) × ⋯ × 3 × 2 × 1 

For example, 

    5! = 5 * 4 * 3 * 2 ( 1 = 120
    
Write a function that computes the factorial of any given number. Include argument validity checking