{-# LANGUAGE ScopedTypeVariables #-}
module Main where
import Data.List
import Control.Monad

readLine :: Read a => IO [a]
readLine = map read . words <$> getLine

check :: Int -> Int -> Bool
check s p = elem s $ map ((`mod` 12) . (+p)) [0,2,4,5,7,9,11]

calc :: [Int] -> Int -> Bool
calc t p
    | p == -1 = False
    | null t = True
    | check (head t) p = calc (tail t) p
    | otherwise = False

main :: IO ()
main = do
    getLine
    t <- readLine
    let ans = [p | p<-[0..11], calc t p]
    print $ if length ans == 1 then head ans else -1