module Main where
import Control.Monad
import Control.Applicative
import Data.Maybe
import Data.List
import qualified Text.Printf
import qualified Data.ByteString.Char8 as BC
import qualified Data.ByteString as BS

import Debug.Trace

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data Tile = Front | Back deriving (Eq, Show)

fromChar :: Char -> Tile
fromChar '#' = Front
fromChar _ = Back

flipTile Front = Back
flipTile Back  = Front

main :: IO ()
main = do
  n <- readInt
  s <- map fromChar <$> BC.unpack <$> BC.getLine
  let leng = length s
      res = min (solve (n-1) s) (solve (leng-n) $ reverse s)
  print res

solve :: Int -> [Tile] -> Int
solve n ts' =
  let ts = foldr arrange [] $ dropWhile (== Front) (mapn n flipTile ts')
        where arrange Back [] =[]
              arrange x xs = x:xs
      n0 = n - min n (length ts' - length ts)
      score acc [] = acc
      score acc (Back:[]) = acc
      score acc (x:y:[]) = 
        case (x, y) of
          (Back, Back)   -> acc
          (Back, Front)  -> acc+1
          (Front, Front) -> acc+2
          (Front, Back)  -> acc+3
      score acc (x:y:ys) = 
        case x of
          Back  -> score (acc+1) (flipTile y:ys)
          Front -> score (acc+3) (y:ys)
  in  score n0 ts

mapn :: Int -> (a -> a) -> [a] -> [a]
mapn _ _ [] = []
mapn n f (x:xs)
  | n <= 0    = x:xs
  | otherwise = f x : mapn (n-1) f xs

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{- Int input -}

parseInt :: BC.ByteString -> Int
parseInt = fst . fromJust . BC.readInt

parseInts :: BC.ByteString -> [Int]
parseInts = map parseInt <$> BC.words

readInt :: IO Int
readInt = parseInt <$> BC.getLine

readInts :: IO [Int]
readInts = parseInts <$> BC.getLine

{- Double Formatting -}

doubleFmt :: Double -> String
doubleFmt = Text.Printf.printf "%.12f"