#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>
#include <numeric>
#include <functional>
#include <cmath>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <sstream>
#include <string>
#define repd(i,a,b) for (int i=(int)(a);i<(int)(b);i++)
#define rep(i,n) repd(i,0,n)
#define all(x) (x).begin(),(x).end()
#define mod 1000000007
#define inf 2000000007
#define mp make_pair
#define pb push_back
typedef long long ll;
using namespace std;
template <typename T>
inline void output(T a, int p) {
    if(p) cout << fixed << setprecision(p)  << a << "\n";
    else cout << a << "\n";
}
// end of template

int dx[8] = {1, 1, 1, 0, 0, -1, -1, -1};
int dy[8] = {1, 0, -1, 1, -1, 1, 0, -1};

int main() {
    cin.tie(0);
    ios::sync_with_stdio(0);
    // source code
    int H, W;
    cin >> H >> W;
    vector<string> A(H);
    rep(i, H) cin >> A[i];
    vector<vector<int>> B(H + 2, vector<int>(W + 2, -1));
    
    vector<pair<int, int>> st;
    rep(i, H + 2){
        B[i][0] = 0, st.pb(mp(i, 0));
        B[i][W + 1] = 0, st.pb(mp(i, W + 1));
    }
    rep(i, W + 2){
        B[0][i] = 0, st.pb(mp(0, i));
        B[H + 1][i] = 0, st.pb(mp(H + 1, i));
    }
    rep(i, H) rep(j, W){
        if(A[i][j] == '.') B[i + 1][j + 1] = 0, st.pb(mp(i + 1, j + 1));
    }
    
    int cur = 0;
    while(!st.empty()){
        cur++;
        vector<pair<int, int>> tmp;
        for(auto itr: st) {
            int y = itr.first, x = itr.second;
            rep(i, 8){
                int ny = y + dy[i], nx = x + dx[i];
                if(ny >= 0 && ny < H + 2 && nx >= 0 && nx < W + 2){
                    if(B[ny][nx] == -1){
                        B[ny][nx] = cur;
                        tmp.pb(mp(ny, nx));
                    }
                }
            }
        }
        st = tmp;
    }
    
    output(cur - 1, 0);
    return 0;
}