{-# OPTIONS_GHC -O2 #-}
import qualified Control.Arrow         as Arrow
import qualified Data.List             as List
import qualified Data.ByteString.Char8 as BSC8
import qualified Data.Maybe            as Maybe

readInts :: IO [Int]
readInts = map (fst . Maybe.fromJust . BSC8.readInt) . BSC8.words <$> BSC8.getLine

main :: IO ()
main = do
  BSC8.getLine
  xs <- readInts
  print $ solve xs

solve :: [Int] -> Int
solve xs = if l > 1 || l == 0
            then -1
            else g $ filter snd ys
  where
    tables = map major xs
    ys = map (head Arrow.&&& ok xs) tables
    l = length $ filter snd ys
    g [(a, b)] = a

ok :: [Int] -> [Int] -> Bool
ok [] _ = True
ok (x:xs) ys = (List.elem x ys) && (ok xs ys)

major :: Int -> [Int]
major n = [(n + x) `mod` 12 | x <- [0, 2, 4, 5, 7, 9, 11]]