input: Write a function to calculate the greatest common divisor (GCD) of two numbers. The function should return the GCD, and when the function is called with two specific numbers, it should display the result.
  
  def gcd(a, b):
      if b == 0: 
          return a 
      return gcd(b, a % b) 
  
  assert gcd(10, 20) == 10
  assert gcd(25, 5) == 5
  assert gcd(48, 18) == 6
  assert gcd(15, 45) == 15
  assert gcd(14, 28) == 14
output: def gcd(a, b):
      if b == 0: 
          return a 
      return gcd(b, a % b)