(*
ocamlfind ocamlopt -package batteries -linkpkg main.ml -o a.out
*)
open Batteries

let slst = Str.split (Str.regexp "") (read_line ())

let rec f lst =
        match lst with
                  [] -> "yes"
                | first :: rest -> 
                        let exists = List.exists (fun elmnt -> elmnt = first) rest in
                                if exists then
                                        "no"
                                else
                                        f rest

let () =
        f slst
        |> Printf.printf "%s"
