#define _USE_MATH_DEFINES
#include <cstdio>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <complex>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <limits>
#include <climits>
#include <cfloat>
#include <functional>
#include <iterator>
using namespace std;

int main()
{
    int h, w;
    cin >> h >> w;
    vector<bitset<3002> > s(h+2);
    for(int y=1; y<=h; ++y){
        for(int x=1; x<=w; ++x){
            char c;
            cin >> c;
            if(c == '#')
                s[y][x] = true;
        }
    }

    int ans = 0;
    for(;;){
        ++ ans;
        for(int y=1; y<=h; ++y)
            s[y] = s[y] & (s[y] << 1) & (s[y] >> 1);

        vector<bitset<3002> > t(h+2);
        bool isRest = false;
        for(int y=1; y<=h; ++y){
            t[y] = s[y] & s[y-1] & s[y+1];
            isRest |= t[y].any();
        }
        s.swap(t);

        if(!isRest){
            cout << ans << endl;
            return 0;
        }
    }
}