from collections import deque
import sys
input = sys.stdin.readline
sys.setrecursionlimit(10 ** 7)
DX = (-1, 0, 1, 0, -1, -1, 1, 1)
DY = (0, 1, 0, -1, -1, 1, -1, 1)
R = 1500


H, W = map(int, input().split())
G = [input().rstrip() for _ in range(H)]
dist = [[R]*W for _ in range(H)]

X = deque()
Y = deque()
D = deque()
for i in range(H):
    for j in range(W):
        if G[i][j] == ".":
            dist[i][j] = 0
            X.appendleft(i)
            Y.appendleft(j)
            D.appendleft(0)
        elif i == 0 or i == H - 1 or j == 0 or j == W - 1:
            dist[i][j] = 1
            X.append(i)
            Y.append(j)
            D.append(1)

while X:
    x = X.popleft()
    y = Y.popleft()
    now = D.popleft()
    for dx, dy in zip(DX, DY):
        nx = x + dx
        ny = y + dy
        if 0 <= nx < H and 0 <= ny < W and dist[nx][ny] > now + 1:
            dist[nx][ny] = now + 1
            X.append(nx)
            Y.append(ny)
            D.append(now + 1)

print(now)