#!/usr/bin/env pypy3

import array
import heapq
import itertools

INF = 10 ** 8


def compute(height, width, is_sea):
    dist = [array.array("L", (INF for _ in range(width)))
            for _ in range(height)]
    pq = []
    for r, c in itertools.product(range(height), range(width)):
        if is_sea[r][c]:
            dist[r][c] = 0
            pq.append((0, (r, c)))
    heapq.heapify(pq)
    while pq:
        _, (r0, c0) = heapq.heappop(pq)
        for dr, dc in itertools.product((1, 0, -1), repeat=2):
            if dr == dc == 0:
                continue
            r, c = r0 + dr, c0 + dc
            if r < 0 or r >= height or c < 0 or c >= height:
                continue
            if is_sea[r][c]:
                continue
            new_length = dist[r0][c0] + 1
            if new_length < dist[r][c]:
                dist[r][c] = new_length
                heapq.heappush(pq, (new_length, (r, c)))
    rc = itertools.product(range(height), range(width))
    return max(dist[r][c] for r, c in rc)


def main():
    height, width = map(int, input().split())
    is_sea = [array.array("B", (True for _ in range(width + 2)))]
    for _ in range(height):
        row = array.array("B", (True, ))
        row.extend(map(lambda x: x == ".", input()))
        row.append(True)
        is_sea.append(row)
    is_sea.append(array.array("B", (True for _ in range(width + 2))))
    print(compute(height + 2, width + 2, is_sea))


if __name__ == '__main__':
    main()