solveEquations :: [[Int]] -> Int
solveEquations pairs
    | all (== ans1) anss && ans1 > 0 = ans1
    | otherwise                      = -1
    where
        anss = map (\(x : y : _) -> y - x) pairs
        ans1 = head anss

main :: IO ()
main = do
    _ <- getLine
    print . solveEquations . map (map read) . map words . lines =<< getContents