import java.util.LinkedList;
import java.util.Queue;
import java.util.Scanner;

public class Yuki402 {
	static int H, W;
	static String[] S;
	static int[][] mp;

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		H = sc.nextInt();
		W = sc.nextInt();
		S = new String[H + 2];
		mp = new int[H + 2][W + 2];
		sc.nextLine();
		for (int i = 0; i < H + 2; i++) {
			if (i == 0 || i == H + 1) {
				S[i] = str(W + 2, '.');
			} else {
				S[i] = sc.nextLine();
				S[i] = '.' + S[i] + '.';
			}
		}
		System.out.println(Search());
		sc.close();
	}

	static int Search() {
		int res = 0;
		int[] dh = new int[] { 0, 0, -1, -1, -1, 1, 1, 1 };
		int[] dw = new int[] { 1, -1, 1, -1, 0, 1, -1, 0 };
		Queue<pair> q = new LinkedList<pair>();
		for (int i = 0; i < H + 2; i++) {
			for (int j = 0; j < W + 2; j++) {
				mp[i][j] = 0;
				if (S[i].charAt(j) == '.') {
					mp[i][j] = -1;
					q.add(new pair(i, j));
				}
			}
		}
		while (!q.isEmpty()) {
			pair buf = q.poll();
			int h = buf.getFirst();
			int w = buf.getSecond();
			for (int i = 0; i < 8; i++) {
				int nh = h + dh[i];
				int nw = w + dw[i];
				if (nh < 0 || nh >= H + 2)
					continue;
				if (nw < 0 || nw >= W + 2)
					continue;
				if (mp[nh][nw] == -1)
					continue;
				if (mp[h][w] == -1) {
					mp[nh][nw] = 1;
					q.add(new pair(nh, nw));
				} else if (mp[nh][nw] > mp[h][w] + 1 || mp[nh][nw] == 0) {
					mp[nh][nw] = mp[h][w] + 1;
					q.add(new pair(nh, nw));
				}
				res = Math.max(res, mp[nh][nw]);
			}
		}
		return res;
	}

	static String str(int n, char c) {
		String res = "";
		for (int i = 0; i < n; i++) {
			res += c;
		}
		return res;
	}

	static class pair {
		private int first;
		private int second;

		pair(int f, int s) {
			this.first = f;
			this.second = s;
		}

		int getFirst() {
			return this.first;
		}

		int getSecond() {
			return this.second;
		}
	};
}