{-# LANGUAGE ScopedTypeVariables #-}

import Data.Array.IO
import Data.Array
import Data.IORef
import Control.Monad
import Control.Applicative

paint :: IOUArray (Int, Int) Char -> [(Int,Int)] -> [(Int,Int)] -> IO [(Int,Int)]
paint grid [] newq = return newq
paint grid (p@(y,x):queue) newq = do
  ((miny,minx),(maxy,maxx)) <- getBounds grid
  rd <- readArray grid p
  writeArray grid p '#'
  next <- filterM (\(ny,nx) ->
            if (ny < miny || nx < minx || ny > maxy || nx > maxx) then
              return False
            else do
              ng <- readArray grid (ny,nx)
              return (ng /= '#' && (ny,nx) `notElem` newq)
            ) [(y-1,x),(y+1,x),(y,x-1),(y,x+1)]
  paint grid queue (newq ++ next)

solve :: IOUArray (Int, Int) Char -> [(Int,Int)] -> Int -> IO Int
solve _ [] cnt = return cnt
solve grid black cnt = do
  black' <- paint grid black []
  solve grid black' (cnt+1)

main = do
  [h,w] <- map read . words <$> getLine :: IO [Int]
  strs <- replicateM h getLine :: IO [String]
  (grid :: IOUArray (Int, Int) Char) <- thaw $ listArray ((1,1),(h,w)) $ concat strs
  grid' <- freeze grid
  let black = [i | (i,e) <- assocs grid', e == '#']
  res <- solve grid black (-1) 
  print res