import java.util.Scanner;

public class Yuki402 {
	static Scanner sc = new Scanner(System.in);

	public static void main(String[] args) {
		int H, W;
		H = sc.nextInt();
		W = sc.nextInt();
		sc.nextLine();
		int[][] dp = new int[H][W];
		int ans = 0;
		for (int i = 0; i < H; i++) {
			String s = sc.nextLine();
			for (int j = 0; j < W; j++) {
				if (s.charAt(j) == '.') {
					dp[i][j] = 0;
				} else {
					dp[i][j] = (int) 1e9;
					if (i - 1 >= 0)
						dp[i][j] = Math.min(dp[i][j], dp[i - 1][j]);
					if (j - 1 >= 0)
						dp[i][j] = Math.min(dp[i][j], dp[i][j - 1]);
					if (i - 1 >= 0 && j - 1 >= 0)
						dp[i][j] = Math.min(dp[i][j], dp[i - 1][j - 1]);
					dp[i][j]++;
					ans = Math.max(ans, dp[i][j]);
				}
			}
		}
		ans = (ans + 1) / 2;
		System.out.println(ans);
	}
}