#include <bits/stdc++.h>
#define rep(i, n) for(int i=0; i<(n); ++i)
#define rep2(i, s, n) for(int i=s; i<(n); ++i)
#define ALL(v) (v).begin(), (v).end()

using namespace std;

typedef long long ll;

template<typename T>
using priority_queue_rev = priority_queue<T, vector<T>, greater<T> >;

static const int INTINF = (2147483647 >> 1); // 10^9 + 5000
static const ll LLINF = (9223372036854775807 >> 1);
static const int MAX = 1e5+1;
static const ll MOD = 1e9+7;

namespace Kunitaka{
    template<
        typename TYPE,
        std::size_t SIZE
    >
    std::size_t array_length(const TYPE (&array)[SIZE])
    {
        return SIZE;
    }

    template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
    template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
}
using namespace Kunitaka;
namespace Printer{
    void br(){
        cout << endl;
    }

    void loop_cnt(int i){
        cout << "i = " << i << endl;
    }

    void loop_cnt(int i, int j){
        cout << "i = " << i << ", " << "j = " << j << endl;
    }

    template<class T>
    void print(T x){
        cout << x << endl;
    }

    template<class T, class S>
    void print(T x, S y){
        cout << x << "," << y << endl;
    }

    template<class T, class S, class U>
    void print(T x, S y, U z){
        cout << x << "," << y << "," << z << endl;
    }

    template<class T, class S>
    void print(pair<T, S> p){
        cout << p.first << ", " << p.second << endl;
    }

    template<
        typename TYPE,
        std::size_t SIZE
    >
    void print(const TYPE (&array)[SIZE]){
        int lim = array_length(array);
        for(int i=0; i<lim; i++){
            if(i) cout << " ";
            cout << array[i];
        }
        cout << endl;
    }

    template<class S>
    void print(vector<S> v){
        for(int i=0; i<v.size(); i++){
            if(i) cout << " ";
            cout<< v[i];
        }
        cout << endl;
    }

    template<class S>
    void print(vector<vector<S> > vv){
        for(int i=0; i<vv.size(); i++){
            for(int j=0; j<vv[i].size(); j++){
                if(j) cout << " ";
                cout << vv[i][j];
            }
            cout << endl;
        }
    }

    void yesno(bool x){
        if(x) cout << "Yes" << endl;
        else cout << "No" << endl;
    }
};
using namespace Printer;

struct point{
    int x, y;
};
int d[1001][1001];
int dx[4] = {0, 0, -1, 1};
int dy[4] = {-1, 1, 0, 0};

int main(int argc, const char * argv[]) {
    //提出時、消す----//


    // input
    cout << fixed << setprecision(10);
    int H, W; cin >> H >> W;
    string vv[1001];
    rep(i, H){
        cin >> vv[i];
    }

    queue<point> q;
    memset(d, -1, sizeof(d));

    rep(i, H){
        rep(j, W){
            if(vv[i][j] == '.') continue;
            if(d[i][j] != -1) continue;
            d[i][j] = 0;
            q.push({j, i});
        }
    }

    while(!q.empty()){
        point now = q.front(); q.pop();
        for(int i=0; i<4; i++){
            point ne = {now.x, now.y};
            ne.x += dx[i]; ne.y += dy[i];
            if(ne.x < 0 || W <= ne.x) continue;
            if(ne.y < 0 || H <= ne.y) continue;
            if(vv[ne.y][ne.x] == '#') continue;
            if(d[ne.y][ne.x] != -1) continue;
            d[ne.y][ne.x] = d[now.y][now.x] + 1;
            q.push({ne.x, ne.y});
        }
    }

    int ans = 0;
    rep(i, H){
        rep(j, W){
            // if(j) cout << " ";
            // cout << d[i][j];
            if(d[i][j] == -1) continue;
            ans = max(ans, d[i][j]);
        }
        // cout << endl;
    }



    cout << ans << endl;
    
    
    return 0;
}


Measured memory_bytes: 13456