input: Write a function to calculate the greatest common divisor of two numbers and display the result. If either number is zero, return zero.
  assert gcd(10, 20)==10
  assert gcd(24, 36)==12
  assert gcd(0, 5)==0
  assert gcd(7, 0)==0
  assert gcd(0, 0)==0
output: def gcd(a, b):
      if a == 0 or b == 0:
          return 0
      while b != 0:
          a, b = b, a % b
      return a