#include <limits>
#include <iostream>
#include <algorithm>
#include <iomanip>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <numeric>
#include <bitset>
#include <cmath>

static const int MOD = 1000000007;
using ll = long long;
using u32 = uint32_t;
using namespace std;

template<class T> constexpr T INF = ::numeric_limits<T>::max() / 32 * 15 + 208;

int main() {
    int h, w;
    cin >> h >> w;
    vector<vector<int>> d(h+4, vector<int> (w+4, -1));
    queue<pair<int, int>> p;
    for (int i = 0; i <= h+1; ++i) {
        for (int j = 0; j <= w+1; ++j) {
            if(i == 0 || j == 0 || i == h+1 || j == w+1) {
                d[i+1][j+1] = 0;
                p.emplace(i, j);
            }
        }
    }
    for (int i = 0; i < h; ++i) {
        string s;
        cin >> s;
        for (int j = 0; j < w; ++j) {
            d[i+2][j+2] = (s[j] == '.' ? 0 : INF<int>);
            if(s[j] == '.'){
                p.emplace(i+1, j+1);
            }
        }
    }
    int ans = 0;
    while(!p.empty()){
        int i, j; tie(i, j) = p.front(); p.pop();
        for (int dy = -1; dy <= 1; ++dy) {
            for (int dx = -1; dx <= 1; ++dx) {
                if(!dy && !dx) continue;
                if(d[i+1+dx][j+1+dy] == -1) continue;
                if(d[i+1+dx][j+1+dy] > d[i+1][j+1]+1){
                    p.emplace(i+dx, j+dy);
                    d[i+1+dx][j+1+dy] = d[i+1][j+1]+1;
                    ans = max(ans, d[i+1][j+1]+1);
                }
            }
        }
    }
    cout << ans << "\n";
    return 0;
}