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

public static partial class Program
{
    public static void Main()
    {
        var hw = CRL().ArrayInts();
        var h = hw[0];
        var w = hw[1];
        var map = new int[h, w];
        for (int i = 0; i < h; i++)
        {
            var str = CRL();
            for (int j = 0; j < w; j++)
            {
                if (str[j] == '#')
                {
                    map[i, j] = 1;
                }
            }
        }

        var queue = new Queue<KeyValuePair<int, int>>();

        //最初の一回目
        for (int row = 0; row < h; row++)
        {
            for (int col = 0; col < w; col++)
            {
                if (map[row, col] == 1)
                {
                    queue.Enqueue(new KeyValuePair<int, int>(row, col));
                }
            }
        }

        var cnt = 0;
        while (true)
        {
            var bufq = new Queue<KeyValuePair<int, int>>();
            while (queue.Count != 0)
            {
                var q = queue.Dequeue();

                //4方向
                for (int i = 0; i < 4; i++)
                {
                    var y = q.Key + dy[i];
                    var x = q.Value + dx[i];
                    if (y < 0 || y >= h || x < 0 || x >= w)
                    {
                        continue;
                    }

                    if (map[y, x] == 0)
                    {
                        map[y, x] = 1;
                        bufq.Enqueue(new KeyValuePair<int, int>(y, x));
                    }
                }
            }

            queue = bufq;

            if (queue.Count == 0)
            {
                CWL(cnt);
                return;
            }

            cnt++;
        }
    }
}

public static partial class Program
{
    //テンプレここから
    public static int ToInt(this string str) => int.Parse(str);
    public static long ToLong(this string str) => long.Parse(str);
    public static long[] ToLong(this IEnumerable<string> strs) => strs.Select(x => x.ToLong()).ToArray();
    public static int[] ToInt(this IEnumerable<string> strs) => strs.Select(x => x.ToInt()).ToArray();
    public static long[] ArrayLongs(this string str, char opr = ' ') => str.Split(opr).ToLong();
    public static int[] ArrayInts(this string str, char opr = ' ') => str.Split(opr).ToInt();
    public static string CRL() => Console.ReadLine();
    public static void CWL(object obj) => Console.WriteLine(obj);
    public static long Diff(long a, long b) => Math.Abs(Math.Max(a, b) - Math.Min(a, b));
    public static long Sign(this long i) => Math.Sign(i);
    public static bool IsEven(this long i) => i % 2 == 0;
    public static bool IsEven(this int i) => i % 2 == 0;
    public static int[] dx = new[] {0, -1, 0, 1};
    public static int[] dy = new[] {-1, 0, 1, 0};
    public static long mod = 1000000007;
    public static string alphabet = "abcdefghijklmnopqrstuvwxyz";
    public static long ToLong(this char c) => c.ToString().ToInt();

    public static long[] Sort(this long[] array) => array.OrderBy(x => x).ToArray();
    public static int[] Sort(this int[] array) => array.OrderBy(x => x).ToArray();
    public static long[] DSort(this long[] array) => array.OrderByDescending(x => x).ToArray();
    public static int[] DSort(this int[] array) => array.OrderByDescending(x => x).ToArray();

    public static long GCD(long a, long b) => b == 0 ? a : GCD(b, a % b);

    public static long LCM(long a, long b) => a * b / GCD(a, b);

    //テンプレここまで
}

Measured memory_bytes: 35416