import Control.Applicative

main :: IO ()
main = solve <$> f >>= print
  where
    f = map read <$> words <$> getLine

solve :: [Int] -> Int
solve [a, b] = f . divs $ a + b
  where
    f [] = -1
    f (c:cs)
      | a /= c && b /= c && (a + c) `mod` b == 0 && (b + c) `mod` a == 0 = c
      | otherwise = f cs


divs :: Int -> [Int]
divs n = let (xs, ys) = f [] [] 1
         in xs ++ ys
  where
    f as bs x
      | x ^ 2 > n = (reverse as, bs)
      | n `mod` x == 0 = f (x:as) (n `div` x :bs) (x + 1)
      | otherwise = f as bs (x + 1)