import qualified Data.ByteString.Char8 as B
import Data.Maybe
import Data.Functor

readInts :: B.ByteString -> [Int]
readInts x = fst <$> mapMaybe B.readInt (B.words x)

getBeans :: B.ByteString -> Int
getBeans x = (x' !! 0) * (x' !! 1)
  where x' = readInts x

getSum :: B.ByteString -> Int
getSum x = foldr (+) 0 x'
  where x' = readInts x

main :: IO ()
main =
  getBeans <$> B.getLine >>= \ a ->
    getSum <$> B.getLine >>= \ b ->
      (putStrLn . show) (if a >= b then a - b else -1)