{-# LANGUAGE ScopedTypeVariables #-}

import Data.Array.IO
import Data.Array
import Data.IORef
import Data.List
import Control.Monad
import Control.Applicative
import qualified Data.Sequence as Seq

paint :: IOUArray (Int, Int) Char -> Seq.Seq (Int,Int) -> Seq.Seq (Int,Int) -> IO (Seq.Seq (Int, Int))
paint grid queue newq = 
  case Seq.viewl queue of
    Seq.EmptyL -> return newq
    ((y,x) Seq.:< queue') -> do
      ((miny,minx),(maxy,maxx)) <- getBounds grid
      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 /= '#')
                ) [(y-1,x),(y+1,x),(y,x-1),(y,x+1)]
      forM_ next $ \n -> writeArray grid n '#'
      paint grid queue' (foldl' (Seq.|>) newq next) 

solve :: IOUArray (Int, Int) Char -> Seq.Seq (Int,Int) -> Int -> IO Int
solve grid black cnt 
  | Seq.null black = return cnt
  | otherwise = do
    black' <- paint grid black Seq.empty 
    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
  as <- getAssocs grid
  let black = [i | (i,e) <- as, e == '#']
  res <- solve grid (foldl' (Seq.|>) Seq.empty black) (-1) 
  print res