input: Write a function to find the GCD of two numbers. If either of the numbers is non-positive, return -1.
  def findGCD(a, b): 
     if a <= 0 or b <= 0:
         return -1
     if (b == 0): 
         return a
     return findGCD(b, a % b)
  assert findGCD(1000, 500) == 500
  assert findGCD(36, 48) == 12
  assert findGCD(0, 7) == -1
  assert findGCD(12, -3) == -1
  assert findGCD(15, 25) == 5
output: def findGCD(a, b): 
     if a <= 0 or b <= 0:
         return -1
     if (b == 0): 
         return a
     return findGCD(b, a % b)