import Control.Monad

wafuu :: [[Int]] -> Int
wafuu (x:xs)
    | wafuu' a xs && a > 0  = a
    | otherwise             = (-1)
    where
        a = foldr (-) 0 $ reverse x

wafuu' :: Int -> [[Int]] -> Bool
wafuu' n [] = True
wafuu' n (y:ys) =
    n == (foldr (-) 0 $ reverse y) && wafuu' n ys

main = do
    n <- readLn
    str <- replicateM n getLine
    let
        xs = map (map read . words) str :: [[Int]]
    print $ wafuu xs