input: Write a function to find the GCD of two numbers. The function should take two integers as parameters and return their greatest common divisor. If either of the numbers is negative, the function should return -1 as the GCD cannot be negative.
  def findGCD(a, b): 
     if b == 0: 
         return a
     return findGCD(b, a % b)
  assert findGCD(1000, 500) == 500
  assert findGCD(24, 18) == 6
  assert findGCD(8, 12) == 4
  assert findGCD(17, 13) == 1
  assert findGCD(-10, 5) == -1
  assert findGCD(0, 0) == 0
output: def findGCD(a, b): 
      if a < 0 or b < 0:
          return -1
      if b == 0: 
          return a
      return findGCD(b, a % b)