import Control.Applicative ((<$>), (<*>))
import Data.List (group)

main :: IO ()
main = solve <$> readLn <*> getLine <*> getLine >>= print

solve :: Int -> String -> String -> Int
solve d s1 s2 = f 0 ls
  where ls = 0 : ((map length . group) ((replicate 14 'x') ++ s1 ++ s2 ++ (replicate 14 'x'))) ++ [0]
        f mc (o1:x:o2:xs) | x <= d = f (max mc (o1 + x + o2)) (o2:xs)
                          | otherwise = f (max mc ((max o1 o2) + d)) (o2:xs)
        f mc _ = mc