import Data.Functor

mame :: Int -> [Int] -> Int
mame s x = if result >= 0 then result else -1
    where result = s - sum x

main = do
    str1 <- getLine
    str2 <- getLine
    let
        [k, n, f] = map read $ words str1 :: [Int]
        alist = map read $ words str2 :: [Int]
    print $ mame (k*n) alist