import collections
H,W = map(int,input().split())
H+=2
W+=2
MAP = []
MAP.append(list([0]*W))
for _ in range(H-2):
    S = list(input())
    L = [0]*W
    for i in range(1,W-1):
        if S[i-1]=='#':
            L[i]=1
    MAP.append(L)
MAP.append(list([0]*W))

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

deqx = collections.deque()
deqy = 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]==1:
            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]==0:
                    deqx.append(x)
                    deqy.append(y)
cnt = -1
while len(deqx):
    cnt+=1
    ndeqx = collections.deque()
    ndeqy = collections.deque()
    while len(deqx):
        i = deqx.pop()
        j = deqy.pop()
        MAP[i][j]=0
        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]==1:
                MAP[x][y]=0
                ndeqx.append(x)
                ndeqy.append(y)
    deqx = ndeqx
    deqy = ndeqy
print(cnt)