import sys
input = sys.stdin.readline

n, m = map(int, input().split())
grid = []
grid.append([False] * (m + 2))
for _ in range(n):
    grid.append([False] + list(map(lambda x: True if x == "X" else False, input().strip())))
    grid[-1].append(False)
grid.append([False] * (m + 2))

# for r in grid:
#     print([1 if x else 0 for x in r])

import heapq
frontier = [(0, 0, 0)]

depths = [[float('inf') for j in range(m + 2)] for i in range(n + 2)]
while len(frontier) > 0:
    depth, i, j = heapq.heappop(frontier)

    if depth >= depths[i][j]:
        continue

    depths[i][j] = depth

    for di, dj in [[-1, 0], [1, 0], [0, -1], [0, 1]]:
        if i + di < 0 or j + dj < 0 or i + di >= n + 2 or j + dj >= m + 2:
            continue
        heapq.heappush(frontier, ((depth + 1 if grid[i + di][j + dj] else 0), i + di, j + dj))

# print()
m = 0
for r in depths:
    # print(r)
    m = max(max(r), m)

print(m)