#include <bits/stdc++.h>
using namespace std;
#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;

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

int main() {
    cin.tie(0)->sync_with_stdio(0);
    cin.exceptions(cin.failbit);
    int n, m; cin >> n >> m;
    vector<string> grid(n);
    for (int i = 0; i < n; i++) {
        cin >> grid[i];
    }
    vector<vector<int>> dist(n, vector<int>(m));
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            if (grid[i][j] != 'X') continue;
            if (i == 0 || i == n - 1 || j == 0 || j == m - 1) {
                dist[i][j] = 1;
            } else {
                for (int k = 0; k < 4; k++) {
                    int nx = i + dx[k];
                    int ny = j + dy[k];
                    if (grid[nx][ny] != 'X') dist[i][j] = 1;
                }
            }
        }
    }
    queue<array<int, 2>> bfs;
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            if (dist[i][j] == 1) {
                bfs.push({i, j});
            }
        }
    }
    while (!bfs.empty()) {
        auto [x, y] = bfs.front(); bfs.pop();
        for (int i = 0; i < 4; i++) {
            int nx = x + dx[i];
            int ny = y + dy[i];
            if (nx >= 0 && ny >= 0 && nx < n && ny < m && dist[nx][ny] == 0 && grid[nx][ny] == 'X') {
                dist[nx][ny] = dist[x][y] + 1;
                bfs.push({nx, ny});
            }
        }
    }
    int ans = 0;
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            ans = max(ans, dist[i][j]);
        }
    }
    cout << ans << '\n';
}