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

int W, H;
int dist[3010][3010] = { 0 };
char s[3010];

int main(void)
{
    cin >> H >> W;
    queue<int> Q;

    for (int i = 2; i <= H + 1; i++) {
        scanf("%s", s);
        for (int j = 2; j <= W + 1; j++) {
            if (s[j - 2] == '#') {
                dist[i][j] = 1510;
            }
            else {
                Q.push(0); Q.push(j); Q.push(i);
            }
        }
    }

    for (int i = 2; i <= H + 1; i++) {
        Q.push(0); Q.push(1); Q.push(i);
        Q.push(0); Q.push(W + 2); Q.push(i);
    }
    for (int i = 2; i <= W + 1; i++) {
        Q.push(0); Q.push(i); Q.push(1);
        Q.push(0); Q.push(i); Q.push(H + 2);
    }

    int dx[] = { 0, 1, 0, -1, 1, 1, -1, -1 };
    int dy[] = { 1, 0, -1, 0, 1, -1, 1, -1 };
    int ans = 0;

    while (Q.size()) {
        int t = Q.front(); Q.pop();
        int x = Q.front(); Q.pop();
        int y = Q.front(); Q.pop();
        for (int i = 0; i < 8; i++) {
            int nx = x + dx[i];
            int ny = y + dy[i];
            if (dist[ny][nx] > t + 1) {
                ans = max(ans, dist[ny][nx] = t + 1);
                Q.push(t + 1); Q.push(nx); Q.push(ny);
            }
        }
    }

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