#include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <utility>
#include <string>
#include <queue>
#include <tuple>

using namespace std;

typedef tuple<int, int, int> TUP;

const vector<int> dx = {1, 1,  0, -1, -1, -1,  0,  1};
const vector<int> dy = {0, 1,  1,  1,  0, -1, -1, -1};
const int INF = 1e9;

void show(vector<vector<int>> &v) {
cout <<"---------" << endl;
	for (auto vv : v) {
		for (auto vvv : vv) {
			if (vvv == INF) {
				printf("- ");
			} else {
				printf("%d ", vvv);
			}
		}
		printf("\n");
	}
}

int main() {
	int h, w;

	cin >> h >> w;
	vector<string> s(h);
	for (int i = 0; i < h; i++) {
		cin >> s[i];
	}

	vector<vector<int>> t(h, vector<int>(w,INF));
	queue<TUP> que;

	for (int i = 0; i < h; i++) {
		que.push(make_tuple(-1, i, 0));
		que.push(make_tuple( w, i, 0));
	}

	for (int i = 0; i < w; i++) {
		que.push(make_tuple(i, -1, 0));
		que.push(make_tuple(i,  h, 0));
	}

	for (int i = 0; i < h; i++) {
		for (int j = 0; j < w; j++) {
			if (s[i][j] == '.') {
				que.push(make_tuple(j, i, 0));
				t[i][j] = 0;
			}
		}
	}

	int ans = 0;
	while (!que.empty()) {
		auto tup = que.front();
		que.pop();
		int x = get<0>(tup);
		int y = get<1>(tup);
		int v = get<2>(tup);
		int nv = v + 1;

		for (int i = 0; i < 8; i++) {
			int nx = x + dx[i];
			int ny = y + dy[i];
			if (nx < 0 || w <= nx || ny < 0 || h <= ny) {
				continue;
			}
			if (nv < t[ny][nx]) {
				t[ny][nx] = nv;
				que.push(make_tuple(nx, ny, nv));
				ans = max(ans, nv);
			}
		}
	}
	// show(t);
	cout << ans << endl;
	return 0;
}