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

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

    int h, w;
    cin >> h >> w;

    queue<pair<int16_t, int16_t>> q;
    for (int i = 2; i < h + 2; i++) {
        q.push({ i, 1 });
        q.push({ i, w + 2 });
    }
    for (int j = 2; j < w + 2; j++) {
        q.push({ 1, j });
        q.push({ h + 2, j });
    }

    vector<vector<int16_t>> a(h + 4, vector<int16_t>(w + 4, 0));
    string s;
    for (int i = 0; i < h; i++) {
        cin >> s;
        for (int j = 0; j < w; j++) {
            if (s[j] == '#') {
                a[i + 2][j + 2] = 1 << 14;
            } else {
                q.push({ i + 2, j + 2 });
            }
        }
    }

    int r = 0;
    while (!q.empty()) {
        auto p = q.front(); q.pop();
        int d = a[p.first][p.second] + 1;
        for (int i = p.first - 1; i <= p.first + 1; i++) {
            for (int j = p.second - 1; j <= p.second + 1; j++) {
                if (a[i][j] > d) {
                    r = a[i][j] = d;
                    q.push({ i, j });
                }
            }
        }
    }

    cout << r << endl;

    return 0;
}