let arr_of_str_i s = Array.init (String.length s) (fun i -> i, s.[i])
let () =
  let s = Scanf.scanf "%s" arr_of_str_i in
  let b = s |> Array.map (fun (i, c) -> Array.for_all (fun (j, d) -> i = j || c <> d) s)
  |> Array.for_all (fun b -> b) in
  print_endline @@ if b then "yes" else "no"
