from collections import deque

h, w = map(int, input().split())
S = ["." * (w + 2)] + ["." + input() + "." for _ in range(h)] + ["." * (w + 2)]
h += 2
w += 2
queue = deque()
dist = [[-1] * w for _ in range(h)]
for i in range(h):
    for j in range(w):
        if S[i][j] == ".":
            dist[i][j] = 0
            queue.append((i, j))

ans = 0
while queue:
    i, j = queue.popleft()
    for di in range(-1, 2):
        ni = i + di
        if ni == -1 or ni == h:
            continue
        for dj in range(-1, 2):
            nj = j + dj
            if nj == -1 or nj == w:
                continue
            if dist[ni][nj] != -1:
                continue
            dist[ni][nj] = dist[i][j] + 1
            ans = dist[ni][nj]
            queue.append((ni, nj))

print(ans)