main = getLine >> getContents >>= print . solve . map (map read . words) . lines

solve :: [[Int]] -> Int
solve xs
    | same xs' && head xs' > 0 = head xs'
    | otherwise = -1
        where
            xs' = map (\[x, y] -> y - x) xs
            same (y : ys) = foldl (\acc x -> y == x && acc) True ys