input: Write a function to calculate the greatest common divisor (GCD) of two numbers and return the result. The function should be recursive and use the Euclidean algorithm. If the second number is zero, the GCD is the first number. Otherwise, the GCD of two numbers a and b is equal to the GCD of b and a modulo b.
  
  def gcd(a, b):
      if b == 0:
          return a
      return gcd(b, a % b)
  
  assert gcd(10, 20)==10
  assert gcd(9, 18)==9
  assert gcd(15, 25)==5
  assert gcd(30, 45)==15
  assert gcd(8, 13)==1
output: def gcd(a, b):
      if b == 0:
          return a
      return gcd(b, a % b)