input: Write a function to calculate the greatest common divisor (GCD) of two numbers and return the result. If either of the numbers is zero, return 0.
  def gcd(a, b):
      if a == 0 or b == 0:
          return 0
      while b:
          a, b = b, a % b
      return a
  
  assert gcd(10, 20)==10
  assert gcd(24, 36)==12
  assert gcd(0, 5)==0
  assert gcd(15, 0)==0
  assert gcd(0, 0)==0
output: def gcd(a, b):
      if a == 0 or b == 0:
          return 0
      while b:
          a, b = b, a % b
      return a