import java.util.Arrays;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.Scanner;
import java.util.Set;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		final int H = sc.nextInt();
		final int W = sc.nextInt();
		
		boolean[][] is_block = new boolean[H + 2][W + 2];
		for(int i = 0; i < H; i++){
			final char[] input = sc.next().toCharArray();
			
			for(int j = 0; j < W; j++){
				is_block[i + 1][j + 1] = input[j] == '#';
			}
		}
		
		final int INF = Integer.MAX_VALUE / 2;
		int[][] DP = new int[H + 2][W + 2];
		for(int i = 0; i < DP.length; i++){
			for(int j = 0; j < DP[i].length; j++){
				DP[i][j] = is_block[i][j] ? INF : 0;
			}
		}
		
		for(int i = 1; i <= H; i++){
			for(int j = 1; j <= W; j++){
				DP[i][j] = Math.min(DP[i][j], Math.min(Math.min(DP[i - 1][j], DP[i][j - 1]), Math.min(DP[i - 1][j - 1], DP[i - 1][j + 1])) + 1);
			}
		}
		
		for(int i = H; i >= 1; i--){
			for(int j = W; j >= 1; j--){
				DP[i][j] = Math.min(DP[i][j], Math.min(Math.min(DP[i + 1][j], DP[i][j + 1]), Math.min(DP[i + 1][j + 1], DP[i + 1][j - 1])) + 1);
			}
		}
		
		int max = 0;
		for(int i = 0; i < DP.length; i++){
			for(int j = 0; j < DP[i].length; j++){
				max = Math.max(max, DP[i][j]);
			}
		}
		
		System.out.println(max);
	}
}