input: Write a function to calculate the greatest common divisor (GCD) of two numbers using the Euclidean algorithm.
  assert gcd(24, 32) == 8
  assert gcd(48, 18) == 6
  assert gcd(101, 103) == 1
output: def gcd(a, b):
      while b != 0:
          a, b = b, a % b
      return a