// Original C++ solution (slower):
#include <iostream>

#include <string>

#include <cstring>

#include <fstream>

#include <random>

#include <cmath>

#include <iomanip>

#include <cstdlib>

#include <algorithm>

#include <vector>

#include <map>

#include <deque>

#include <map>

#include <set>

#include <queue>

#include <stack>

#include <list>

#include <unordered_map>

#include <bitset>

#include <sstream>



typedef  long long ll;

#define mop 1000000007

using namespace std;



struct point{

    ll x,y,p;

};

int main() {

    



    bool lis[1001][1001] = {};

    ll h,w;

    cin >> h >> w;

    queue <point> v;

    for (ll i=0;i<h;i++){

        string s;

        cin >> s;

        for (ll j=0;j<w;j++){

            if(s[j] == '#'){

                lis[i+1][j+1] = true;

                point tmp;

                tmp.x = i+1;

                tmp.y = j+1;

                tmp.p = 0;

                v.push(tmp);

            }

        }

    }

    ll ans = 0;

    while(!v.empty()){

        point tmp;

        tmp = v.front();

        ans = max(ans, tmp.p);

//        cout << tmp.x << " " << tmp.y << " " << tmp.p << " " << ans << endl;

        v.pop();

        point tmp2;

        if(tmp.x - 1 >= 1 and lis[tmp.x-1][tmp.y] == false){

            lis[tmp.x-1][tmp.y] = true;

            tmp2.x = tmp.x-1;

            tmp2.y = tmp.y;

            tmp2.p = tmp.p+1;

            v.push(tmp2);

        }

        if(tmp.x + 1 <= h and lis[tmp.x+1][tmp.y] == false){

            lis[tmp.x+1][tmp.y] = true;

            tmp2.x = tmp.x+1;

            tmp2.y = tmp.y;

            tmp2.p = tmp.p+1;

            v.push(tmp2);

        }

        if(tmp.y - 1 >= 1 and lis[tmp.x][tmp.y-1] == false){

            lis[tmp.x][tmp.y-1] = true;

            tmp2.x = tmp.x;

            tmp2.y = tmp.y-1;

            tmp2.p = tmp.p+1;

            v.push(tmp2);

        }

        if(tmp.y + 1 <= w and lis[tmp.x][tmp.y+1] == false){

            lis[tmp.x][tmp.y+1] = true;

            tmp2.x = tmp.x;

            tmp2.y = tmp.y+1;

            tmp2.p = tmp.p+1;

            v.push(tmp2);

        }

    }

    cout << ans << endl;



}

// Optimized solution to the same problem (faster):
#include <iostream>

#include <string>

#include <cstring>

#include <fstream>

#include <random>

#include <cmath>

#include <iomanip>

#include <climits>

#include <cstdlib>

#include <algorithm>

#include <vector>

#include <map>

#include <deque>

#include <map>

#include <set>

#include <queue>

#include <stack>

#include <list>

#include <unordered_map>

#include <bitset>

#include <sstream>

#include <new>

#include <typeinfo>

#include <iterator>



typedef long long ll;

typedef unsigned long long ull;

constexpr ll mop = 1000000007;

constexpr ll mop2 = 998244353;



using namespace std;



struct Point {

	ll x, y;

	Point(){}

	Point(ll x, ll y):x(x), y(y){}

};

int main() {

	ll h, w;

	cin >> h >> w;

	queue<Point> blacks;

	vector<vector<ll>> distance;

	distance.resize(h);

	for (ll i = 0; i < h; i++) {

		string s;

		cin >> s;

		distance[i].resize(w);

		for (ll j = 0; j < w; j++) {

			if (s[j] == '#') {

				blacks.push(Point(i, j));

				distance[i][j] = 0;

			}

			else {

				distance[i][j] = -1;

			}

		}

	}

	ll ans = 0;

	while (!blacks.empty()) {

		Point now = blacks.front();

		blacks.pop();

		ll x = now.x;

		ll y = now.y;

		ll nowDis = distance[x][y];

		if (ans < nowDis) {

			ans = nowDis;

		}



		Point tmp[4];

		for (ll i = 0; i < 4; i++) {

			tmp[i] = now;

		}

		tmp[0].x++;

		tmp[1].x--;

		tmp[2].y++;

		tmp[3].y--;

		for (ll i = 0; i < 4; i++) {

			Point& nowtmp = tmp[i];

			if (0 <= nowtmp.x && nowtmp.x < h && 0 <= nowtmp.y && nowtmp.y < w && distance[nowtmp.x][nowtmp.y] == -1) {

				distance[nowtmp.x][nowtmp.y] = nowDis + 1;

				

				blacks.push(nowtmp);

			}

		}

	}

	/*for (ll i = 0; i < h; i++) {

		for (ll j = 0; j < w; j++) {

			cout << distance[i][j];

		}

		cout << endl;

	}*/

	cout << ans << endl;

	

}


// Fastest known solution for this problem: