input: Write a function called `gcd` that takes two integers as input and calculates their greatest common divisor using the Euclidean algorithm. The function should return the greatest common divisor. If the function works correctly, it should produce the output `The greatest common divisor of 10 and 20 is 10` when called with 10 and 20.
  
  def gcd(a, b):
      if b == 0: 
          return a 
      return gcd(b, a % b) 
    
  num1 = 10
  num2 = 20
  
  gcd_value = gcd(num1, num2)
  
  print(f'The greatest common divisor of {num1} and {num2} is {gcd_value}')
  assert gcd(10, 20) == 10
  assert gcd(10, 21) == 1
  assert gcd(25, 15) == 5
  assert gcd(100, 25) == 25
output: def gcd(a, b):
      if b == 0: 
          return a 
      return gcd(b, a % b)