input: Write a python function to calculate the greatest common divisor of two numbers.
  assert greatest_common_divisor(54, 24) == 6
  assert greatest_common_divisor(48, 18) == 6
  assert greatest_common_divisor(101, 103) == 1
output: def greatest_common_divisor(a, b):
      while b:
          a, b = b, a % b
      return a