module Main where

import Control.Applicative
import Control.Monad

main :: IO ()
main = do
    [k, n, f] <- fmap read . words <$> getLine
    as <- fmap read . words <$> getLine
    let mame = k * n :: Int
    let eat  = sum as
    case mame - eat of
        r | r < 0     -> print (-1)
          | otherwise -> print r