import Prelude

main :: IO ()
main = do
  [k, n, _] <- map read . words <$> getLine
  as <- map read . words <$> getLine
  print $ calc k n as

calc :: Int -> Int -> [Int] -> Int
calc k n as = let ms = k * n
                  ts = sum as
              in if ms < ts then -1 else ms - ts