#include <iostream>
#include <vector>
#include <cstring>
#include <string>
#include <algorithm>
#include <iomanip>
#include <queue>
using namespace std;

const int INF = 1000000;
char b[3010][3010];
int d[3010][3010];
int dx[8] = { 0, 1, 1, 1, 0, -1, -1, -1 }, dy[8] = { -1, -1, 0, 1, 1, 1, 0, -1 };

struct P {
	int x, y;
};

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	int H, W;
	cin >> H >> W;

	fill((int*)begin(d), (int*)end(d), INF);

	queue<P> q;
	for(int i = 0; i < H + 2; i++) {
		for(int j = 0; j < W + 2; j++) {
			if(i != 0 && i != H + 1 && j != 0 && j != W + 1) {
				cin >> b[i][j];
			}
			else {
				b[i][j] = '.';
			}
			if(b[i][j] == '.') {
				d[i][j] = 0;
				q.push(P{ j, i });
			}
		}
	}

	int ans = 0;
	while(!q.empty()) {
		P p = q.front();
		q.pop();

		for(int i = 0; i < 8; i++) {
			int ny = p.y + dy[i], nx = p.x + dx[i];
			if(1 <= nx && nx <= W && 1 <= ny && ny <= H && d[p.y][p.x] + 1 < d[ny][nx]) {
				d[ny][nx] = d[p.y][p.x] + 1;
				q.push(P{ nx, ny });
				ans = max(ans, d[ny][nx]);
			}
		}
	}

	cout << ans << endl;
}