let solve n abArr =
  let a4bArr = Array.map (fun (a, b) -> a + 4 * b) abArr in
  Array.sort compare a4bArr;
  let maxAb = a4bArr.(n - 1) in
  let rec solve' idx result =
    if idx >= n then result
    else let c2 = maxAb - a4bArr.(idx) in
         if c2 mod 2 <> 0 then -1
         else solve' (idx + 1) (c2 / 2 + result)
  in
  solve' 0 0

let () =
  let n = read_int () in
  let abArr = Array.make n (0, 0) in
  let rec readAb i =
    if i < n then (Scanf.scanf "%d %d\n" (fun a b -> abArr.(i) <- (a, b));
                   readAb (i + 1))
  in
  readAb 0;
  solve n abArr |> print_int;
  print_newline ()