#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;

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

    int r = 1;
    for (int i = 1; i < h; i++) {
        for (int j = 1; j < w; j++) {
            if (a[i][j] == 0) continue;
            a[i][j] = min(min(a[i - 1][j - 1], a[i - 1][j]), a[i][j - 1]) + 1;
            r = max(r, a[i][j]);
        }
    }

    cout << (r + 1) / 2 << endl;

    return 0;
}