#include <iostream>
#include <stdio.h>
#include <algorithm>
using namespace std;

int W, H;
int dp[3010][3010] = { 0 };
char c[3010];

int main(void)
{
    cin >> H >> W;

    int res = 0;
    for (int i = 1; i <= H; i++) {
        scanf("%s", c);
        for (int j = 1; j <= W; j++) {
            if (c[j - 1] == '#') res = max(res, dp[i][j] = min(dp[i - 1][j - 1], min(dp[i - 1][j], dp[i][j - 1])) + 1);
        }
    }
    cout << (res + 1) / 2 << endl;
    return 0;
}