{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE BangPatterns #-}

import Control.Applicative
import Control.Monad
import Control.Arrow
import Data.List
import Data.Maybe
import Data.Char
import Text.Printf

readInt = ( readLn :: IO Int )
readInts = map ( read :: String -> Int ) . words <$> getLine

which a b f = if f then a else b
mp [ a, b ] = ( a, b )

main = do
	[ k, n, f ] <- readInts
	s <- sum <$> readInts

	print $ which ( k * n - s ) ( -1 ) ( s <= k * n )