import Data.List
main::IO()
main= do
  n <- fmap read getLine
  xs <- fmap (map (map read . words)) . sequence . replicate n $ getLine
  print . (\xs-> let a = head xs 
                 in if ((a>0) && ((find (/= a) xs) == Nothing))
                    then a else (-1))
        . map (\(a:b:[])->b-a) $xs