import Control.Applicative ((<$>), (<*>))

main :: IO ()
main = do
  [n, m] <- map read <$> words <$> getLine
  solve n m <$> getLine <*> getLine >>= print

solve :: Int -> Int -> String -> String -> Int
solve n m s t = let (sa, sb) = f n s
                    (ta, tb) = f m t
                in min sa ta + min sb tb
  where f l xs = let l' = (length . filter (== 'A')) xs in (l', l - l')