Scanf.scanf "%s" (fun s ->
    let n = String.length s in
    
    let rec loop i pos acc =
        if i = n then acc else
            if s.[i] = 'A' then (
                loop (i + 1) (pos + 1) (i - pos + acc)
            ) else
                loop (i + 1) pos acc
    in
    loop 0 0 0 |> Printf.printf "%d\n"
)