import Data.List
import Control.Applicative
import Control.Monad

main = do
     n <- readLn
     c <- (map . map) (read :: String -> Int) . map words <$> replicateM n getLine
     let c' = map (\x -> x !! 1 - x !! 0) c
         s  = (head c) !! 1 - (head c) !! 0 
     print $ if and $ map (== s) c'
               then if s < 0
                       then -1
                       else s
               else -1