Scanf.scanf "%d" (fun n ->
    let xy = Array.init n (fun _ -> Scanf.scanf " %d %d" (fun x y -> y - x)) in
    let mi, mx = Array.fold_left (fun (mi, mx) v -> min mi v, max mx v) (max_int, min_int) xy in
    Printf.printf "%d\n" @@ if mi = mx && mi > 0 then mi else (-1)
)