//
//
//  Codeforces: ganariya
//  AtCoder: ganariya2525
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//  MMMMMMMMMMMMMMMMNOtXtdMMMMMNxzZMMNmgHyVVVVVW%..?/(.,(x-..........(HfffpffpffpffWHHffpffppfVyyWMMMMMM


//include
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <unordered_map>
#include <climits>
#include <set>
#include <unordered_set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <queue>
#include <random>
#include <complex>
#include <regex>
#include <locale>
#include <random>
#include <type_traits>

using namespace std;

#define SHOW_VECTOR(v) {std::cerr << #v << "\t:";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOW_MAP(v){std::cerr << #v << endl; for(const auto& xxx: v){std::cerr << xxx.first << " " << xxx.second << "\n";}}

using LL = long long;

//~~~~~~~~~~~~~~~~~~~~~_(＾～＾ ｣ ∠)_~~~~~~~~~~~~~~~~~~~~~

constexpr int INF = 1e7;

int main() {

    int H, W;
    cin >> H >> W;

    vector<string> A(H);
    for (int i = 0; i < H; i++) cin >> A[i];

    queue<pair<int, int>> Q;
    vector<vector<int>> I(H, vector<int>(W, INF));
    for (int i = 0; i < H; i++) {
        for (int j = 0; j < W; j++) {
            if (A[i][j] == '#') {
                I[i][j] = 0;
                Q.push(make_pair(i, j));
            }
        }
    }

    vector<int> dy = {0, 1, 0, -1};
    vector<int> dx = {1, 0, -1, 0};
    while (!Q.empty()) {
        auto p = Q.front();
        Q.pop();
        int y = p.first;
        int x = p.second;
        for (int i = 0; i < 4; i++) {
            int toY = y + dy[i];
            int toX = x + dx[i];
            if (toY < 0 || toY >= H || toX < 0 || toX >= W) continue;
            if (A[toY][toX] == '#') continue;
            if (I[toY][toX] == INF) {
                I[toY][toX] = I[y][x] + 1;
                A[toY][toX] = '#';
                Q.push(make_pair(toY, toX));
            }
        }
    }

    int ans = 0;
    for (int i = 0; i < H; i++) for (int j = 0; j < W; j++) ans = max(ans, I[i][j]);

    cout << ans << endl;

    return 0;
}










































Measured memory_bytes: 13584