import Data.Maybe (fromJust)
import qualified Data.ByteString.Char8 as BSC8

readInts :: IO [Integer]
readInts = map (fst . fromJust . BSC8.readInteger) . BSC8.words <$> BSC8.getLine

main :: IO ()
main = do
  (s:k:_) <- readInts
  print $ if (s - k) < 0 then -1 else s - k