input: Write a python function to calculate the greatest common divisor (GCD) of two numbers using recursion.
  assert GCD(98, 56) == 14
  assert GCD(48, 18) == 6
  assert GCD(101, 103) == 1
output: def GCD(a, b):
      if b == 0:
          return a
      else:
          return GCD(b, a % b)