### BUGGY CODE ###

## coding: UTF-8
from decimal import *
from itertools import permutations, combinations,combinations_with_replacement,product
import math
#import collections
#from math import gcd 旧環境でこれを有効化するとREを吐きます
#import sys
#input = sys.stdin.readline


def main():
    s = input().split()
    p = [int(w) for w in s] #これで勝手にNまで拾ってくれます
    H = p[0]
    W = p[1]

    '''
    queue = []
    #input = sys.stdin.readline
    A = [list(input()) for _ in range(H)]
    #print('A:{}'.format(A))
    c = [['w'] * (W+2)]
    #print('c:{}'.format(c))
    for i in range(H):
        c.append(['w'])
        for j in range(W):
            if(A[i][j] == '#'):
                queue.append([i+1, j+1])
            c[-1].append(A[i][j])
        c[-1].append('w')
    c.append(['w'] * (W+2))
    #print('c:{}'.format(c))
    #print('queue:{}'.format(queue))
    '''
    
    a = []
    for i in range(H):
        a.append(input())


    #白ベースで最寄りの黒を探す方針
    #パディングは'.'でも'#'でもないもので埋める
    b = ''
    for i in range(W+2):
        b += 'w'
    #print(b)
    #上と下と右と左に空きマスを挿入する
    a.insert(0, b)
    for i in range(1, 1+H):
        a[i] = 'w' + a[i] + 'w'
    a.append(b)


    header = ' '
    for i in range(len(a[0])):
        header += ('{}'.format(i))

    c=[]
    for i in range(H+2):
        c.append([])
    queue = []
    for i in range(H+2):
        for j in range(W+2):
            if(a[i][j] == '#'):
                queue.append([i, j])
            c[i].append(a[i][j])


    def see_banmen():
        print(header)
        for i in range(len(c)):
            print('{}{}'.format(i,''.join(c[i])))
    #see_banmen()
    


    #queue = [start]
    next_list = [] #次の歩数で探すものを格納しておく
    step = 1
    #mode = 0
    #answer_mode = 0
    status = True #ゴール判定でゴールについていたらFalseにする
    #answer = -1




    '''
    for i in range(H+2):
        for j in range(W+2):
            if(c[i][j] == '#'):
                queue.append([i, j])
    '''
    while status:
        if(len(queue) > 0):
            pivot = queue.pop(0) #軸足はqueueの先頭
            x = pivot[0]
            y = pivot[1]
            if(c[x+1][y] == '.'):
                c[x+1][y] = 'd'
                next_list.append([x+1, y])
            if(c[x-1][y] == '.'):
                c[x-1][y] = 'd'
                next_list.append([x-1, y])
            if(c[x][y+1] == '.'):
                c[x][y+1] = 'd'
                next_list.append([x, y+1])
            if(c[x][y-1] == '.'):
                c[x][y-1] = 'd'
                next_list.append([x, y-1])
        else:
            #queueの中身が空っぽになった
            #基本的にはstep += 1
            if(len(next_list) > 0):
                #next_listの中身がある場合（まだ探索が終了していない場合）
                #next_list -> queue にして next_listはからにする
                '''
                for i in range(len(next_list)):
                    queue.append(next_list.pop(0))
                next_list = []
                '''
                queue = next_list
                next_list = []
                

                #print()
                #print('step{}end'.format(step))
                #see_banmen()
                #次の探索に移る
                step += 1
            else:
                #初めてnext_listの中身がなくなった場合（探索は終了したい）
                #もう進めない
                #print()
                #print('もう進めない')
                #print('step{}end'.format(step))
                #see_banmen()
                status = False

    print(step - 1)
            
if __name__ == '__main__':
    main()
            

### FIXED CODE ###

## coding: UTF-8


H,W = map(int,input().split())

c = []
for i in range(H):
    c.append(input())

fresh = [True] * (H*W)
#print(fresh)


queue = []
n = []
for i in range(H):
    for j in range(W):
        if(c[i][j] == '#'):
            fresh[i*W+j] = False
            queue.append(i*W+j)
#print('initial')
#print('fresh:{}'.format(fresh))
#print('queue:{}'.format(queue))

step = 0
status = True
while status:
    if(len(queue)>0):
        #処理を行う
        pivot = queue.pop()
        #print('pivot:{}, queue:{}, n:{}'.format(pivot, queue, n))        
        #右へ移動
        if(pivot%W != W-1 and fresh[pivot+1]):
            n.append(pivot+1)
            fresh[pivot+1] = False
        #左へ移動
        if(pivot%W != 0 and fresh[pivot-1]):
            n.append(pivot-1)
            fresh[pivot-1] = False
        #下へ移動
        if( int(pivot/W)<(H-1) and fresh[pivot+W]):
            n.append(pivot+W)
            fresh[pivot+W] = False
        #上へ移動
        if( int(pivot/W)>0 and fresh[pivot-W] ):
            n.append(pivot-W)
            fresh[pivot-W] = False
        #print('pivot:{}, queue:{}, n:{}'.format(pivot, queue, n))
    elif(len(n)>0):
        queue = n
        n = []
        #歩数を増やそう
        step += 1
        #print('step{}end,queue:{}, n:{}'.format(step, queue, n))
    else:
        status = False


print(step)