using System;
using System.Collections.Generic;
using System.Text;
using System.Linq;


class Program
{
    public void Proc()
    {
        Reader.IsDebug = false;
        int[] inpt = Reader.ReadLine().Split(' ').Select(a => int.Parse(a)).ToArray();

        int h = inpt[0];
        int w = inpt[1];

        char[,] map = new char[h, w];
        int[,] step = new int[h, w];
        int maxHen = 0;
        for (int i = 0; i < inpt[0]; i++)
        {
            string row = Reader.ReadLine();
            for (int j = 0; j < inpt[1]; j++)
            {
                map[i, j] = row[j];
                if (row[j] == '#')
                {
                    int val = 1;
                    if (i == 0 || i == h - 1 || j == 0 || j == w - 1)
                    {
                    }
                    else
                    {
                        val = Math.Min(step[i - 1, j - 1] + 1, Math.Min(step[i, j - 1] + 1, step[i - 1, j] + 1));
                    }
                    step[i, j] = val;
                    maxHen = Math.Max(maxHen, val);
                }
            }
        }

        Console.WriteLine((maxHen + 1) / 2);
    }




    public class Reader
    {
        public static bool IsDebug = true;
        private static String PlainInput = @"


8 8
.#..#...
.######.
#####...
#####.#.
########
######.#
#####...
...####.




";
        private static System.IO.StringReader Sr = null;
        public static string ReadLine()
        {
            if (IsDebug)
            {
                if (Sr == null)
                {
                    Sr = new System.IO.StringReader(PlainInput.Trim());
                }
                return Sr.ReadLine();
            }
            else
            {
                return Console.ReadLine();
            }
        }
    }
    static void Main()
    {
        Program prg = new Program();
        prg.Proc();
    }
}