solve :: Integral a => a -> a -> a
solve n m
    | odd n = if n <= m then 1 else -1
    | n <= m = 1
    | d <= m = 2
    | otherwise = -1
    where d = div n 2

main :: IO ()
main = do
    [n, m] <- fmap read . words <$> getLine :: IO [Int]
    print $ solve n m