let () =
  Scanf.scanf "%d\n" @@ fun n ->
  let arr = Array.init n @@ fun _ -> Scanf.scanf "%d %d\n" @@ fun x y -> (x, y) in
  let ans = (snd arr.(0)) - (fst arr.(0)) in
  let b = Array.for_all (fun (x, y) -> y - x = ans) arr in
  Printf.printf "%d\n" (if b && ans > 0 then ans else -1)