In mathematics, the greatest common divisor (gcd) of two or more integers, which are not all zero, is the largest positive integer that divides each of the integers.

For example, the gcd of 8 and 12 is 4.

A simple recursive algorithm for calculating this is the Euclidean algorithm which can be described as:

gcd(a, a)=a

gcd(a, b)=gcd(a-b, b) if a>b

gcd(a, b)=gcd(a, b-a) if a

In python, write a function gcd() to calculate the gcd using this algorithm and test using 8 and 12.