W, H = map(int, input().split())
grid = [input() for i in range(H)]
dist = [[-1] * W for i in range(H)]

import heapq
dr = [-1, 0, 0, 1]
dc = [0, -1, 1, 0]

sr = -1
sc = -1
for r in range(H):
    for c in range(W):
        if grid[r][c] == ".":
            sr = r
            sc = c
            break
    if sr != -1:
        break

priq = [(0, sr, sc)]
dist[sr][sc] = 0
while len(priq):
    d, r, c = heapq.heappop(priq)
    if d > dist[r][c]:
        continue
    for i in range(4):
        nr = r + dr[i]
        nc = c + dc[i]
        if not (0 <= nr < H and 0 <= nc < W):
            continue
        nd = d
        if grid[nr][nc] == "#":
            nd += 1
        if dist[nr][nc] == -1 or dist[nr][nc] > nd:
            dist[nr][nc] = nd
            heapq.heappush(priq, (nd, nr, nc))

ans = 0
for i in range(H):
    for j in range(W):
        if grid[i][j] == ".":
            ans = max(ans, dist[i][j])
print(ans)