import Control.Applicative ((<$>))

main :: IO ()
main = do
  [k, n, _] <- getl $ map read . words
  solve (k * n) <$> getl (sum . map read . words) >>= print

solve :: Int -> Int -> Int
solve c a = if c >= a then c - a else -1

getl :: (String -> a) -> IO a
getl f = f <$> getLine