-- Wrongri-La Shower

{-# LANGUAGE Arrows #-}

import Control.Arrow
import Data.List
    
main = do
  d <- readLn
  s <- getLine
  t <- getLine
  let
      u = (replicate 14 'x') ++ s ++ t ++ (replicate 14 'x')
      f (_:b@(m,'o'):c:ds) = max m $ f (b:c:ds)
      f ((l,_):b@(m,'x'):c@(n,_):ds)
        | m <= d = max (l+m+n) $ f (b:c:ds)
        | otherwise = maximum [d+l, d+n, f (b:c:ds)]
      f _ = 0
   in print $ f $ ((0,'x'):).(++[(0,'x')]) $ map (length &&& head) $ group u