import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		solver();
	}

	static void solver() {
		Scanner sc = new Scanner(System.in);
		int H = sc.nextInt();
		int W = sc.nextInt();
		char[][] table = new char[H][W];
		dis = new int[H][W];

		for (int i = 0; i < H; i++) {
			table[i] = sc.next().toCharArray();
		}

		int[][] dp = new int[H + 2][W + 2];

		int ans = 0;
		for (int i = 1; i <= H; i++) {
			for (int j = 1; j <= W; j++) {
				if (table[i-1][j-1] == '.')
					continue;
				dp[i][j] = Math.min(dp[i - 1][j - 1] + 1, dp[i - 1][j] + 1);
				dp[i][j] = Math.min(dp[i][j], dp[i][j - 1] + 1);

				ans = Math.max(ans, dp[i][j]);
			}
		}
		System.out.println((ans + 1) / 2);

	}

	static int[][] dis;
}