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

//YukiCoder402 最も海から遠い場所
//http://yukicoder.me/problems/no/402
class Program
{
    static string InputPattern = "InputX";

    static List<string> GetInputList()
    {
        var WillReturn = new List<string>();

        if (InputPattern == "Input1") {
            WillReturn.Add("8 8");
            WillReturn.Add(".#..#...");
            WillReturn.Add(".######.");
            WillReturn.Add("#####...");
            WillReturn.Add("#####.#.");
            WillReturn.Add("########");
            WillReturn.Add("######.#");
            WillReturn.Add("#####...");
            WillReturn.Add("...####.");
            //3
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("7 7");
            WillReturn.Add("#######");
            WillReturn.Add("#######");
            WillReturn.Add("#######");
            WillReturn.Add("#######");
            WillReturn.Add("#.#####");
            WillReturn.Add("#..####");
            WillReturn.Add("#######");
            //3
        }
        else if (InputPattern == "Input3") {
            WillReturn.Add("60 60");
            WillReturn.Add("............................................................");
            WillReturn.Add("............................................................");
            WillReturn.Add("........................................#...............##..");
            WillReturn.Add(".......................................####............##...");
            WillReturn.Add(".........................................##...........##....");
            WillReturn.Add(".........................................###........#.......");
            WillReturn.Add(".........................................#####...#..#.......");
            WillReturn.Add(".........................................#########....#.....");
            WillReturn.Add("........................................###########.........");
            WillReturn.Add("........................................#############.......");
            WillReturn.Add(".....................................##.##########..........");
            WillReturn.Add("......................................#########.............");
            WillReturn.Add(".....................................#########..............");
            WillReturn.Add("....................................##.#...###..............");
            WillReturn.Add("...................................###.......#..............");
            WillReturn.Add(".....................................###....................");
            WillReturn.Add(".....................................#......................");
            WillReturn.Add(".......................................##...................");
            WillReturn.Add("......................................#.#...................");
            WillReturn.Add(".....................................####...................");
            WillReturn.Add(".....................................####...................");
            WillReturn.Add(".....................................#####..................");
            WillReturn.Add(".....................................######.................");
            WillReturn.Add(".....................................######.................");
            WillReturn.Add(".....................................######.................");
            WillReturn.Add(".....................................#####..................");
            WillReturn.Add("....................................#####...................");
            WillReturn.Add("....................................#####...................");
            WillReturn.Add("....................................######..................");
            WillReturn.Add("...................................#####....................");
            WillReturn.Add("................................#.######....................");
            WillReturn.Add(".................................#######....................");
            WillReturn.Add("............................#...########....................");
            WillReturn.Add("............................#.##########....................");
            WillReturn.Add("...........................############.....................");
            WillReturn.Add("...........................############.....................");
            WillReturn.Add("..........................#############.....................");
            WillReturn.Add(".........................###############....................");
            WillReturn.Add("....................###..##############.....................");
            WillReturn.Add("...............#####################.##.....................");
            WillReturn.Add("..............###################.#..#......................");
            WillReturn.Add(".....#......###############.####..#.........................");
            WillReturn.Add("..........########......###.................................");
            WillReturn.Add("..........####....##.#.#####................................");
            WillReturn.Add("......#.##...#.######..###..................................");
            WillReturn.Add(".....##.####...######..##...................................");
            WillReturn.Add("......######..####.#........................................");
            WillReturn.Add("...#...######.###...........................................");
            WillReturn.Add("......##.####..#............................................");
            WillReturn.Add(".......#.###................................................");
            WillReturn.Add("........####................................................");
            WillReturn.Add(".......####.................................................");
            WillReturn.Add(".......####.................................................");
            WillReturn.Add(".......#.##.................................................");
            WillReturn.Add("............................................................");
            WillReturn.Add("..........#.................................................");
            WillReturn.Add("........#.#.................................................");
            WillReturn.Add("............................................................");
            WillReturn.Add("............................................................");
            WillReturn.Add("............................................................");
            //4
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    static int UB_X;
    static int UB_Y;

    static void Main()
    {
        List<string> InputList = GetInputList();
        int[] wkArr = InputList[0].Split(' ').Select(X => int.Parse(X)).ToArray();
        int H = wkArr[0];
        int W = wkArr[1];

        UB_X = W - 1;
        UB_Y = H - 1;
        char[,] BanArr = new char[UB_X + 1, UB_Y + 1];

        for (int LoopX = 0; LoopX <= UB_X; LoopX++) {
            for (int LoopY = 0; LoopY <= UB_Y; LoopY++) {
                BanArr[LoopX, LoopY] = InputList[LoopY + 1][LoopX];
            }
        }

        //https://codeiq.jp/magazine/2015/02/21755/
        int[,] CntArr = new int[UB_X + 1, UB_Y + 1];

        Func<int, int, int> wkFunc = (pX, pY) =>
        {
            if (pX < 0 || UB_X < pX) return 0;
            if (pY < 0 || UB_Y < pY) return 0;

            return CntArr[pX, pY];
        };

        for (int LoopX = 0; LoopX <= UB_X; LoopX++) {
            for (int LoopY = 0; LoopY <= UB_Y; LoopY++) {
                if (BanArr[LoopX, LoopY] == '.')
                    CntArr[LoopX, LoopY] = 0;
                else {
                    int Cnt1 = wkFunc(LoopX - 1, LoopY - 1);
                    int Cnt2 = wkFunc(LoopX, LoopY - 1);
                    int Cnt3 = wkFunc(LoopX - 1, LoopY);

                    int wkMin = Math.Min(Cnt1, Cnt2);
                    wkMin = Math.Min(wkMin, Cnt3);

                    CntArr[LoopX, LoopY] = wkMin + 1;
                }
            }
        }

        int MaxSeihoukeiLength = CntArr.Cast<int>().Max();
        if (MaxSeihoukeiLength % 2 == 0) Console.WriteLine(MaxSeihoukeiLength / 2);
        else Console.WriteLine(MaxSeihoukeiLength / 2 + 1);
    }
}