#include <bits/stdc++.h>
#include <iostream>
#include <queue>

using namespace std;

#define rep(i, n) for(int i = 0; i < n; i++)

#define sz(x) (long long)(x).size()

int main() {
    int n, m; cin >> n >> m;
    priority_queue<pair<int, pair<int, int>>> pq;

    pq.push({0, {0, 0}});
    vector<vector<bool>> grid(n+2, vector<bool>(m+2, false));
    rep(i, n) {
        string s; cin >> s;
        rep (j, m) {
            if (s[j]=='X') grid[i+1][j+1] = true;
            else pq.push({0, {i+1, j+1}});
        }
    }

    

    vector<vector<int>> dist(n+2, vector<int>(m+2, INT32_MAX));


    while (sz(pq)) {
        pair<int, pair<int, int>> data = pq.top(); pq.pop();

        int depth = -data.first;
        int r = data.second.first;
        int c = data.second.second;
        if (dist[r][c] <= depth) continue;
        dist[r][c] = depth;
        pair<int, int> disp[4] = {{1, 0}, {0, 1}, {-1, 0}, {0, -1}};
        for (pair<int, int> off : disp) {
            if (r + off.first < 0 or r+off.first >= n+2 or c + off.second < 0 or c+off.second >= m+2) continue;
            pq.push({(grid[r + off.first][c + off.second] ? -depth-1:0), {r+off.first, c+off.second}});
        }
    }
    int mx = 0;
    rep(i, n+2) {rep(j, m+2) {mx = max(mx, dist[i][j]); }}
    cout << mx << endl;
    // rep(i, n+2) {rep(j, m+2) {cout << dist[i][j] << ' ';}cout << endl;}

}