let () =
  let s = read_line () in
  let tbl = Hashtbl.create 26 in
  let sl = String.length s in
  let rec loop i =
    if i = sl then "yes"
    else
      if Hashtbl.mem tbl s.[i] then "no" else begin
          Hashtbl.replace tbl s.[i] true;
          loop (i+1)
        end in
  loop 0 |> print_endline

