defmodule Main do
  defp count(str, x) do
    str
    |> String.graphemes
    |> Enum.count(&(&1 == x))
  end

  def main do
    [_, _] = IO.gets("") |> String.trim |> String.split |> Enum.map(&String.to_integer/1)
    s = IO.gets("") |> String.trim
    t = IO.gets("") |> String.trim

    ans = min(count(s, "A"), count(t, "A")) + min(count(s, "B"), count(t, "B"))

    IO.puts ans
  end
end