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

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

int min4(int a1, int a2, int a3, int a4) {
    return min(a1, min(a2, min(a3, a4)));
}

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

    for (int i = 1; i <= H; i++) {
        scanf("%s", c);
        for (int j = 1; j <= W; j++) {
            if (c[j - 1] == '#') dp[i][j] = 1510;
        }
    }

    for (int i = 1; i <= H; i++) {
        for (int j = 1; j <= W; j++) {
            dp[i][j] = min4(dp[i][j], dp[i - 1][j] + 1, dp[i][j - 1] + 1, dp[i - 1][j - 1] + 1);
        }
    }
    for (int i = H; i >= 1; i--) {
        for (int j = 1; j <= W; j++) {
            dp[i][j] = min4(dp[i][j], dp[i + 1][j] + 1, dp[i][j - 1] + 1, dp[i + 1][j - 1] + 1);
        }
    }
    for (int i = 1; i <= H; i++) {
        for (int j = W; j >= 1; j--) {
            dp[i][j] = min4(dp[i][j], dp[i - 1][j] + 1, dp[i][j + 1] + 1, dp[i - 1][j + 1] + 1);
        }
    }
    for (int i = H; i >= 1; i--) {
        for (int j = W; j >= 1; j--) {
            dp[i][j] = min4(dp[i][j], dp[i + 1][j] + 1, dp[i][j + 1] + 1, dp[i + 1][j + 1] + 1);
        }
    }

    int ans = 0;
    for (int i = 1; i <= H; i++) {
        for (int j = 1; j <= W; j++) {
            ans = max(ans, dp[i][j]);
        }
    }

    cout << ans << endl;
    return 0;
}