let countAB str =
  let len_str = String.length str in
  let str_lst = Array.init len_str (fun i -> str.[i]) |> Array.to_list in
  List.partition (fun c -> c = 'A') str_lst
  |> fun (aLst, bLst) -> (List.length aLst, List.length bLst)

let solve s t =
  let a1, b1 = countAB s
  and a2, b2 = countAB t in
  (min a1 a2) + (min b1 b2)

let () =
  let _ = read_line ()
  and s = read_line ()
  and t = read_line () in
  solve s t
  |> Printf.printf "%d\n"