from collections import deque

w, h = map(int, input().split())
C = [input() for _ in range(h)]
br = False
stack = []
queue = deque()
dist = [[-1] * w for _ in range(h)]
for i in range(h):
    for j in range(w):
        if C[i][j] == ".":
            stack.append((i, j))
            queue.append((i, j))
            dist[i][j] = 0
            br = True
            break
    if br:
        break

directions = [(0, 1), (0, -1), (1, 0), (-1, 0)]
while stack:
    i, j = stack.pop()
    for di, dj in directions:
        ni = i + di
        nj = j + dj
        if ni == -1 or nj == -1 or ni == h or nj == w:
            continue
        if dist[ni][nj] != -1 or C[ni][nj] == "#":
            continue
        dist[ni][nj] = 0
        stack.append((ni, nj))
        queue.append((ni, nj))
        
while queue:
    i, j = queue.popleft()
    for di, dj in directions:
        ni = i + di
        nj = j + dj
        if ni == -1 or nj == -1 or ni == h or nj == w:
            continue
        if dist[ni][nj] != -1:
            continue
        if C[ni][nj] == ".":
            print(dist[i][j])
            queue.clear()
            break
        dist[ni][nj] = dist[i][j] + 1
        queue.append((ni, nj))