module Main where

main :: IO ()
main = getIntList >>= print . solve

solve :: [Int] -> Int
solve [n, m] | n <= m    = 1
             | p1 && p2  = 2
             | otherwise = -1
             where
             p1 = even n
             p2 = n `div` 2 <= m

getIntList :: IO [Int]
getIntList = map read . words <$> getLine