input: Write a function to calculate the greatest common divisor (GCD) of two numbers and display the result.
  def gcd(a, b):
      if b == 0: 
          return a 
      return gcd(b, a % b) 
    
  assert gcd(10, 20)==10
  assert gcd(15, 25)==5
  assert gcd(36, 48)==12
  assert gcd(17, 19)==1
  assert gcd(21, 7)==7
output: def gcd(a, b):
      if b == 0: 
          return a 
      return gcd(b, a % b)