import collections
import copy
H,W = map(int,input().split())
H+=2
W+=2
MAP = []
MAP.append(list('.'*(W)))
for _ in range(H-2):
    MAP.append(list('.'+input()+'.'))
MAP.append(list('.'*(W)))

CNT = [[10000]*W for _ in range(H)]

deq = collections.deque()
X = [0,1,0,-1,1,-1,1,-1]
Y = [1,0,-1,0,1,-1,-1,1]
for i in range(H):
    for j in range(W):
        if MAP[i][j]=='#':
            for k in range(8):
                x = i+X[k]
                y = j+Y[k]
                if 0<=x<H and 0<=y<W and MAP[x][y]=='.':
                    deq.append((x,y))
cnt = -1
while len(deq):
    cnt+=1
    ndeq = collections.deque()
    while len(deq):
        i,j = deq.popleft()
        MAP[i][j]='.'
        for k in range(8):
            x = i+X[k]
            y = j+Y[k]
            if 0<=x<H and 0<=y<W and MAP[x][y]=='#':
                MAP[x][y]='.'
                ndeq.append((x,y))
    deq = ndeq
print(cnt)