input: Write a python function to find the greatest common divisor (GCD) of two numbers using the Euclidean algorithm.
  assert gcd(15, 20) == 5
  assert gcd(100, 25) == 25
  assert gcd(17, 31) == 1
output: def gcd(a, b):
      while b:
          a, b = b, a % b
      return a