import java.util.*;
import java.io.*;

public class Main {
    public static void main(String[] args) throws Exception{
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String[] first = br.readLine().split(" ", 2);
        int h = Integer.parseInt(first[0]);
        int w = Integer.parseInt(first[1]);
        int w2 = w + 2;
        int[][] costs = new int[h + 2][w + 2];
        HashSet<Integer> current = new HashSet<>();
        for (int i = 0; i < w2; i++) {
            current.add(i);
            current.add((h + 1) * w2 + i);
        }
        char[][] field = new char[h][];
        for (int i = 0; i < h; i++) {
            field[i] = br.readLine().toCharArray();
            current.add((i + 1) * w2);
            current.add((i + 1) * w2 + w + 1);
            for (int j = 0; j < w; j++) {
                if (field[i][j] == '.') {
                    current.add((i + 1) * w2 + j + 1);
                }
            }
        }
        HashSet<Integer> used = new HashSet<>();
        used.addAll(current);
        int idx = 0;
        while (used.size() < (h + 2) * (w + 2)) {
            HashSet<Integer> next = new HashSet<>();
            for (int x : current) {
                if (x % w2 > 0) {
                    int tmp = x - 1;
                    if (!used.contains(tmp)) {
                        next.add(tmp);
                    }
                    if (x / w2 > 0) {
                        int tmp2 = tmp - w2;
                        if (!used.contains(tmp2)) {
                            next.add(tmp2);
                        }
                    }
                    if (x / w2 < h + 1) {
                        int tmp2 = tmp + w2;
                        if (!used.contains(tmp2)) {
                            next.add(tmp2);
                        }
                    }
                }
                if (x % w2 < w + 1) {
                    int tmp = x + 1;
                    if (!used.contains(tmp)) {
                        next.add(tmp);
                    }
                    if (x / w2 > 0) {
                        int tmp2 = tmp - w2;
                        if (!used.contains(tmp2)) {
                            next.add(tmp2);
                        }
                    }
                    if (x / w2 < h + 1) {
                        int tmp2 = tmp + w2;
                        if (!used.contains(tmp2)) {
                            next.add(tmp2);
                        }
                    }
               }
                if (x / w2 > 0) {
                    int tmp = x - w2;
                    if (!used.contains(tmp)) {
                        next.add(tmp);
                    }
                }
                if (x / w2 < h + 1) {
                    int tmp = x + w2;
                    if (!used.contains(tmp)) {
                        next.add(tmp);
                    }
                }
            }
            idx++;
            used.addAll(next);
            current = next;
        }
        System.out.println(idx);
    }
    
}