let () =
  let n = Scanf.scanf "%d " (fun i -> i) in
  let a = Array.init n (fun _ -> Scanf.scanf "%d %d " (fun a b -> a + 4*b)) in
  let m = Array.fold_left max 0 a in
  let rec doit i s =
    if i = n then s
    else if a.(i) = m then doit (i + 1) s
    else
      let x = m - a.(i) in
      if x mod 2 <> 0 then (-1)
      else doit (i + 1) (s + x / 2) in
  doit 0 0 |> Printf.printf "%d\n"