solve :: Integer -> Integer -> [Integer] -> String
solve k n as =
  let amount = k * n
      requirement = foldl (+) 0 as
  in show $ max (-1) (amount - requirement)

main :: IO ()
main = do
  [k, n, f] <- (map read . words) <$> getLine
  as <- (map read . words) <$> getLine
  putStr $ solve k n as