using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;
class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static string[] SList(long n) => Enumerable.Repeat(0, (int)n).Select(_ => ReadLine()).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var c = NList;
        var (h, w) = (c[0], c[1]);
        var s = SList(h);

        var cmap = new int[h + 1][];
        for (var i = 0; i < cmap.Length; ++i) cmap[i] = new int[w + 1];
        for (var i = 0; i < h; ++i) for (var j = 0; j < w; ++j)
        {
            cmap[i + 1][j + 1] = cmap[i][j + 1] + cmap[i + 1][j] - cmap[i][j] + (s[i][j] == '#' ? 1 : 0);
        }
        var ok = 1;
        var ng = Math.Min(h, w) + 1;
        while (ng - ok > 1)
        {
            var mid = (ok + ng) / 2;
            var imap = new int[h][];
            for (var i = 0; i < imap.Length; ++i) imap[i] = new int[w];
            for (var i = 0; i + mid <= h; ++i) for (var j = 0; j + mid <= w; ++j)
            {
                var cnt = cmap[i + mid][j + mid] - cmap[i][j + mid] - cmap[i + mid][j] + cmap[i][j];
                if (cnt == mid * mid)
                {
                    ++imap[i][j];
                    if (i + mid < h) --imap[i + mid][j];
                    if (j + mid < w) --imap[i][j + mid];
                    if (i + mid < h && j + mid < w) ++imap[i + mid][j + mid];
                }
            }
            // WriteLine(mid);
            // WriteLine(string.Join("\n", imap.Select(ii => string.Join(" ", ii))));
            for (var i = 0; i < h; ++i) for (var j = 1; j < w; ++j) imap[i][j] += imap[i][j - 1];
            for (var j = 0; j < w; ++j) for (var i = 1; i < h; ++i) imap[i][j] += imap[i - 1][j];
            // WriteLine(string.Join("\n", imap.Select(ii => string.Join(" ", ii))));
            var flg = true;
            for (var i = 0; i < h; ++i) for (var j = 0; j < w; ++j)
            {
                if (s[i][j] == '#' && imap[i][j] == 0) flg = false;
            }
            if (flg) ok = mid;
            else ng = mid;
        }
        WriteLine(ok);
    }


}