input: Write a function to calculate the greatest common divisor (GCD) of two numbers and display the result.
  assert gcd(10, 20) == 10
  assert gcd(27, 15) == 3
  assert gcd(81, 36) == 9
  assert gcd(12, 18) == 6
  
  def gcd(a, b):
      if b == 0:
          return a
      return gcd(b, a % b)
output: def gcd(a, b):
      if b == 0:
          return a
      return gcd(b, a % b)