let () =
  Scanf.scanf "%d\n" @@ fun n ->
  let xrr = Array.init n @@ fun _ -> Scanf.scanf "%d " @@ fun d -> d in
  let () = Array.fast_sort compare xrr in
  let diff = xrr.(1) - xrr.(0) in
  let ans = Array.fold_left 
      (fun (b, bfr) n -> if n - bfr = diff || n = bfr then (b && true, n) else (false, n)) 
      (true, xrr.(0)) xrr in
  Printf.printf "%s\n" (if fst ans then "YES" else "NO")