let () =
  let n = Scanf.scanf "%d " (fun i -> i) in
  let rec doit i z =
    if i = n then z
    else
      let x, y = Scanf.scanf "%d %d " (fun x y -> x, y) in
      if x > y then (-1)
      else
        let s = y - x in
        if z <> (-1) && z <> s then (-1)
        else doit (i + 1) s in
  doit 0 (-1) |> Printf.printf "%d\n"