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
     print $ if and $ map (== (head c')) c'
               then if head c' <= 0
                       then -1
                       else head c'
               else -1