Scanf.scanf "%s" (fun s ->
    let h = Array.make 26 0 in
    String.iter (fun c ->
        let q =  int_of_char c - 65 in
        h.(q) <- h.(q) + 1) s;
    print_endline @@
        if Array.fold_left (fun acc v -> if v > 1 then true else acc) false h then "NO" else "YES"
)