let () =
  Scanf.scanf "%d\n" @@ fun n ->
  let t = Array.init n @@ fun _ -> Scanf.scanf "%d " @@ fun d -> d in
  let check d x =
    if List.mem ((x-d+12) mod 12) [0;2;4;5;7;9;11] then true else false in
  let num = ref [] in
  let i = ref 0 in
  while !i < n do
    let d = t.(!i) in
    let b = Array.fold_left (fun b x -> b && (check d x)) true t in
    if b then num := d :: !num;
    i := !i + 1
  done;
  Printf.printf "%d\n" (if List.length !num = 1 then List.hd !num else -1)