import java.util.Scanner;

class Yuki2_2{	
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);

		int H = sc.nextInt();
		int W = sc.nextInt();
		int dp[][] = new int[H + 2][W + 2];
		int res = 0;
		
		for(int i = 1; i <= H; i++){
		String input = sc.next();
			for(int j = 1; j <= W; j++){
				if(input.charAt(j - 1) == '#'){
					dp[i][j] = Math.min(dp[i - 1][j - 1], Math.min(dp[i - 1][j], dp[i][j - 1])) + 1;
					res = Math.max(res, dp[i][j]);
				}
			}
		}
		System.out.println((res + 1) / 2);
	}
}