import collections
H,W = map(int,input().split())
lsHW = ['.'*(W+2)]+['.'+input()+'.' for i in range(H)]+['.'*(W+2)]
H += 2
W += 2
INF = 5000
cost = [[INF]*(W) for i in range(H)]
used = [[False]*(W) for i in range(H)]
dxy = [(0,1),(1,0),(-1,0),(0,-1),(1,1),(-1,1),(1,-1),(-1,-1)]
d = collections.deque()
for i in range(H):
    for j in range(W):
        if lsHW[i][j] == '.':
            cost[i][j] = 0
            d.append((i,j))

while d:
    x,y = d.popleft()
    if used[x][y]:
        continue
    used[x][y] = True
    for dx,dy in dxy:
        if 0<=x+dx<H and 0<= y+dy <W:
            if used[x+dx][y+dy]:
                continue
            if cost[x+dx][y+dy] >= cost[x][y] + 1:
                cost[x+dx][y+dy] = cost[x][y] + 1
                d.append((x+dx,y+dy))

cmax = 0
for i in range(H):
    for j in range(W):
        cmax = max(cmax,cost[i][j])
print(cmax)