#include <bits/stdc++.h>
using namespace std;
using i64 = int64_t;
using vi = vector<i64>;
using vvi = vector<vi>;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    
    int h, w;
    cin >> h >> w;
    vvi b(h, vi(w, 1e9));
    using ii = pair<int, int>;
    queue<ii> que;
    for (int i = 0; i < h; i++) {
        for (int j = 0; j < w; j++) {
            char c;
            cin >> c;
            if (c == '.') {
                b[i][j] = 0;
                que.push(ii(i, j));
            }
        }
    }

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

    auto ok = [&](int x, int y) {
        return 0 <= x && x < h && 0 <= y && y < w;
    };

    while (que.size()) {
        ii t = que.front();
        que.pop();
        for (int i = 0; i < 8; i++) {
            int x = t.first + dx[i];
            int y = t.second + dy[i];

            if (ok(x, y) && b[t.first][t.second] + 1 < b[x][y]) {
                b[x][y] = b[t.first][t.second] + 1;
                que.push(ii(x, y));
            }
        }
    }

    i64 nax = -1;
    for (int i = 0; i < h; i++) {
        for (int j = 0; j < w; j++) {
            nax = max(nax, b[i][j]);
        }
    }
    
    cout << nax << endl;
}