import Control.Applicative
import Text.Printf

main = do
  [k,n,f] <- fmap (read :: String -> Int) . words <$> getLine
  xs <- fmap (read :: String -> Int) . words <$> getLine
  printf "%d\n" $ solve (k*n) xs
  
solve rest [] = rest
solve rest (x:xs) = if rest < x then -1
                    else solve (rest-x) xs