import Data.List

splitEvery2 :: [Int] -> [[Int]]
splitEvery2 = takeWhile (not . null) . map (take 2) . iterate (drop 2)

main :: IO ()
main = interact $ show . solve . map (\[x, y] -> y - x) . splitEvery2 . map read . tail . words

solve :: [Int] -> Int
solve xs = if t /= 1 then -1 else head xs
    where
        t = length $ group xs