import java.util.Scanner;

class Yuki2_1{	
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);

		int H = sc.nextInt();
		int W = sc.nextInt();
		int map[][] = new int[H + 2][W + 2];

		for(int i = 1; i <= H; i++){
		String input = sc.next();
			for(int j = 1; j <= W; j++){
				if(input.charAt(j - 1) == '#') map[i][j] = 1510;
			}
		}
		
		for(int i = 1; i <= H; i++){
			for(int j = 1; j <= W; j++){
				map[i][j] = min4(map[i][j], map[i - 1][j] + 1, map[i][j - 1] + 1, map[i - 1][j - 1] + 1);
			}
		}
		for(int i = H; i >= 1; i--){
			for(int j = 1; j <= W; j++){
				map[i][j] = min4(map[i][j], map[i + 1][j] + 1, map[i][j - 1] + 1, map[i + 1][j - 1] + 1);
			}
		}
		for(int i = 1; i <= H; i++){
			for(int j = W; j >= 1; j--){
				map[i][j] = min4(map[i][j], map[i - 1][j] + 1, map[i][j + 1] + 1, map[i - 1][j + 1] + 1);
			}
		}
		for(int i = H; i >= 1; i--){
			for(int j = W; j >= 1; j--){
				map[i][j] = min4(map[i][j], map[i + 1][j] + 1, map[i][j + 1] + 1, map[i + 1][j + 1] + 1);
			}
		}
		
		int max = 0;
		for(int i = 1; i <= H; i++){
			for(int j = 1; j <= W; j++){
				max = Math.max(max, map[i][j]);
			}
		}
		
		System.out.println(max);
	}
	
	public static int min4(int a1, int a2, int a3, int a4){
		return Math.min(a1, Math.min(a2, Math.min(a3, a4)));
	}
}