#include <bits/stdc++.h>
using namespace std;

int main() {
	int h, w;
	cin >> h >> w;

	vector<string> g(h + 2, string(w + 2, '.'));
	for (int i = 0; i < h; i++) {
		string s;
		cin >> s;
		for (int j = 0; j < w; j++) {
			g[i + 1][j + 1] = s[j];
		}
	}
	h += 2;
	w += 2;

	vector<vector<int>> dist(h, vector<int>(w, -1));

	queue<pair<int, int>> q;
	for (int i = 0; i < h; i++) {
		for (int j = 0; j < w; j++) {
			if (g[i][j] != '#') {
				dist[i][j] = 0;
				q.emplace(i, j);
			}
		}
	}

	while (!q.empty()) {
		int y, x;
		tie(y, x) = q.front(); q.pop();

		const int dy[] = { 0, 0, 1, 1, 1, -1, -1, -1 };
		const int dx[] = { 1, -1, 1, 0, -1, 1, 0, -1 };

		for (int k = 0; k < 8; k++) {
			int ny = y + dy[k];
			int nx = x + dx[k];

			if (ny < 0 || nx < 0 || ny >= h || nx >= w) continue;
			if (dist[ny][nx] == -1) {
				dist[ny][nx] = dist[y][x] + 1;
				q.emplace(ny, nx);
			}
		}
	}

	int maxi = 0;
	for (int i = 0; i < h; i++) {
		for (int j = 0; j < w; j++) {
			maxi = max(maxi, dist[i][j]);
		}
	}
	cout << maxi << endl;
}