import java.io.PrintWriter;
import java.util.Scanner;

public class Main2 {
    static Scanner scan = new Scanner(System.in);
    static PrintWriter out = new PrintWriter(System.out);

    static int H, W;
    static char[][] map;
    static int [][] ansMap;
    
    static int[] dx = {-1, -1, 0};
    static int[] dy = {0, -1, -1};
    
    public static void main(String[] args) {
        
        String line = scan.nextLine();
        String values[] = line.split(" ");
        H = Integer.parseInt(values[0]);
        W = Integer.parseInt(values[1]);;
        
        map = new char[H][W];
        ansMap = new int[H][W];
        int ans = 0;
        
        for(int i=0; i<H; i++) {
            line = scan.nextLine();
            for(int j=0; j<W; j++) {
                map[i][j] = line.charAt(j);
                if((i==0 || j==0) && map[i][j] == '#') { 
                    ansMap[i][j] = 1;
                    ans = 1;
                }
            }
        }
        
        for(int i=1; i<H; i++) {
            for(int j=1; j<W; j++) {
                if(map[i][j] == '.') continue;
                    
                int v = Integer.MAX_VALUE;
                for(int k=0; k<dx.length; k++) {
                    int nx = j + dx[k];
                    int ny = i + dy[k];
                    v = Math.min(v, ansMap[ny][nx]+1);
                }
                ansMap[i][j] = v;
                ans = Math.max(ans, v);
            }
        }
        
        System.out.println((ans+1)/2);
    }
}