#define _USE_MATH_DEFINES
#include <algorithm>
#include <cstdio>
#include <functional>
#include <iostream>
#include <cfloat>
#include <climits>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <time.h>
#include <unordered_set>
#include <vector>
#include <random>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> i_i;
typedef pair<ll, int> ll_i;
typedef pair<double, int> d_i;
typedef pair<ll, ll> ll_ll;
typedef pair<double, double> d_d;
struct edge { int u, v; ll w; };

int INF = INT_MAX / 2;
ll MOD = 1000000007;
ll _MOD = 1000000009;
double EPS = 1e-10;

char c[3010];

int main() {
	int H, W; cin >> H >> W;
	vector<string> a(H);
	for (int y = 0; y < H; y++) {
		scanf("%s", c);
		a[y] = c;
	}
	queue<i_i> q;
	for (int y = 0; y < H; y++)
		for (int x = 0; x < W; x++)
			if (a[y][x] == '.')
				q.push(i_i(y, x));
	int t;
	for (t = 0; !q.empty(); t++) {
		queue<i_i> _q;
		while (!q.empty()) {
			i_i yx = q.front(); q.pop();
			int y = yx.first, x = yx.second;
			for (int dy = -1; dy <= 1; dy++)
				for (int dx = -1; dx <= 1; dx++) {
					int _y = y + dy, _x = x + dx;
					if (0 <= _y && _y < H && 0 <= _x && _x < W && a[_y][_x] == '#') {
						a[_y][_x] = '.';
						_q.push(i_i(_y, _x));
					}
				}
		}
		q = _q;
	}
	cout << t - 1 << endl;
}