main = do
 getLine
 sts <- map (map read . words) . lines <$> getContents
 print (ws sts)

ws sts = if all even diffs then div (sum diffs) 2 else -1
 where
  diffs = map (flip subtract lmax) lns
  lns = map (\[a,b] -> a + 4*b) sts
  lmax = maximum lns