#!/usr/bin/env python3.8
# %%
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
import numpy as np

# %%
H, W = map(int, readline().split())
S = np.frombuffer(read(), 'S1').reshape(H, -1)[:, :W]

# %%
INF = 10 ** 8
dist = np.zeros((H + 2, W + 2), np.int32)
dist[1:-1, 1:-1] += INF * (S == b'#')

# %%
for _ in range(4):
    dist = dist[::-1].T
    for n in range(1, H + 2):
        np.minimum(dist[n], dist[n - 1] + 1, out=dist[n])
        np.minimum(dist[n, 1:], dist[n - 1, :-1] + 1, out=dist[n, 1:])

# %%
print(dist.max())