Scanf.scanf "%s" (fun s ->
    let n = String.length s in
    let rec loop i acc =
        if i = n then "yes" else
            let c = int_of_char s.[i] - 97 in
            let m = 1 lsl c in
            if m land acc = 0 then loop (i + 1) (acc lor c)
            else "no"
    in
    loop 0 0 |> print_endline
)