import qualified Data.ByteString as BS
import qualified Data.ByteString.Char8 as BC
import Data.Maybe

main :: IO ()
main = do
        n <- (\(x:y:_) -> x * y).r <$> BS.getLine
        x <- foldr (g n) (Just 0).r <$> BS.getLine
        print $ maybe (-1) (n-) x
    where r = map (fst.fromJust.BC.readInt).BC.words
          g n y (Just x) = if x + y > n then Nothing else Just (x + y)
          g _ _ Nothing  = Nothing