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

namespace AGC033A
{
    class Program
    {
        static readonly int[,] xy =
        {
            {0,1 },
            {1,0 },
            {0,-1 },
            {-1,0 },
        };

        static void Solve()
        {
            var h = Input.NextInt();
            var w = Input.NextInt();

            var dist = Enumerable.Repeat(0, h)
                .Select(_ => Enumerable.Repeat(-1, w).ToList())
                .ToList();
            var q = new Queue<Tuple<int, int>>();

            for (int i = 0; i < h; i++)
            {
                var s = Input.Next();
                for (int j = 0; j < w; j++)
                {
                    if (s[j] == '#')
                    {
                        dist[i][j] = 0;
                        q.Enqueue(Tuple.Create(j, i));
                    }
                }
            }

            // bfs
            while (q.Any())
            {
                var current = q.Dequeue();

                for (int i = 0; i < 4; i++)
                {
                    var x = current.Item1 + xy[i, 0];
                    var y = current.Item2 + xy[i, 1];

                    if (x < 0 || y < 0 || x >= w || y >= h) continue;
                    if (dist[y][x] < 0)
                    {
                        dist[y][x] = dist[current.Item2][current.Item1] + 1;
                        q.Enqueue(Tuple.Create(x, y));
                    }
                }
            }

            var max = dist.Max(x => x.Max());

            Console.WriteLine(max);
        }

        #region Competitive Template
        public static void Main(string[] args)
        {
            var needsFlushOutput = true;
            if (needsFlushOutput)
            {
                // 細かく出力しないようにする
                var sw = new System.IO.StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
                Console.SetOut(sw);
            }

            // 仮想的に標準入力をセットする
            // NextLine系を使っていると使えない
            //Input.SetText("");

            Solve();

            Console.Out.Flush();
        }

        static class Input
        {
            static char[] separator = { ' ' };
            public static bool IsEof { get; set; }
            static Queue<string> q { get; set; }

            static Input()
            {
                IsEof = false;
                q = new Queue<string>();
            }

            /// <summary>
            /// 入力予約
            /// </summary>
            /// <param name="items"></param>
            public static void SetText(IEnumerable<string> items)
            {
                foreach (var item in items)
                {
                    SetText(item);
                }
            }

            /// <summary>
            /// 入力予約
            /// </summary>
            /// <param name="s"></param>
            /// <returns></returns>
            public static bool SetText(string s)
            {
                if (s == null) return false;

                foreach (var elem in s.Trim().Split(separator, StringSplitOptions.RemoveEmptyEntries))
                {
                    q.Enqueue(elem);
                }

                return true;
            }

            /// <summary>
            /// 内部queueに入力からの値をsplitして格納する
            /// </summary>
            /// <returns></returns>
            static bool read()
            {
                var s = Console.ReadLine();
                if (s == null) return false;

                foreach (var elem in s.Trim().Split(separator, StringSplitOptions.RemoveEmptyEntries))
                {
                    q.Enqueue(elem);
                }

                if (!q.Any()) return read();

                return true;
            }

            /// <summary>
            /// 次のstringを一つ読み込む
            /// </summary>
            /// <returns></returns>
            public static string Next()
            {
                if (!q.Any())
                {
                    if (!read())
                    {
                        IsEof = true;
                        return "";
                    }
                }

                return q.Dequeue();
            }

            public static int NextInt() => int.Parse(Next());
            public static long NextLong() => long.Parse(Next());
            public static double NextDouble() => double.Parse(Next());

            public static List<string> Next(int n) => Enumerable.Range(0, n).Select(_ => Next()).ToList();
            public static List<int> NextInt(int n) => Next(n).Select(x => int.Parse(x)).ToList();
            public static List<long> NextLong(int n) => Next(n).Select(x => long.Parse(x)).ToList();
            public static List<double> NextDouble(int n) => Next(n).Select(x => double.Parse(x)).ToList();

            public static List<string> NextLine() => Console.ReadLine().Trim().Split(separator, StringSplitOptions.RemoveEmptyEntries).ToList();
            public static List<int> NextIntLine() => NextLine().Select(x => int.Parse(x)).ToList();
            public static List<long> NextLongLine() => NextLine().Select(x => long.Parse(x)).ToList();
            public static List<double> NextDoubleLine() => NextLine().Select(x => double.Parse(x)).ToList();
        }
        #endregion
    }
}


Measured memory_bytes: 54312