input: Write a function to calculate the greatest common divisor (GCD) of two numbers and display the result. If the inputs are 10 and 20, the program should print "The greatest common divisor of 10 and 20 is 10".
  
  def gcd(a, b):
      if b == 0:
          return a
      return gcd(b, a % b)
  
  assert gcd(10, 20) == 10
  assert gcd(5, 7) == 1
  assert gcd(18, 24) == 6
  assert gcd(0, 0) == 0
  assert gcd(15, 0) == 15
output: def gcd(a, b):
      if b == 0:
          return a
      return gcd(b, a % b)