import Control.Applicative ((<$>))
import Control.Monad (replicateM)
import Data.List

main :: IO ()
main = do
  n <- getl read
  solve <$> replicateM n (getl $ map read . words) >>= print

solve :: [[Int]] -> Int
solve = g . map f
  where
    f [x, y] = y - x
    g (x:xs) = if x > 0 && all (== x) xs then x else (-1)

getl :: (String -> a) -> IO a
getl f = f <$> getLine