let solve n st =
  let rec solve' i result =
    if i >= n then
      result
    else (
      let x, y = Stream.next st in
      if (result = -1 && y - x > 0) || (result <> -1 && result = y - x) then
        solve' (i + 1) (y - x)
      else
        -1
    )
  in
  solve' 0 (-1)

let () =
  let n = read_line () |> int_of_string in
  let st = Stream.from(fun i ->
    if i >= n then
      None
    else
      let x, y = read_line ()
                 |> fun l -> Scanf.sscanf l "%d %d" (fun x y -> (x, y))
      in
      Some(x, y))
  in
  solve n st |> string_of_int |> print_endline