using System;
using System.Collections.Generic;

namespace YukiMe2_5 {
    public class Program {
        public static void Main(string[] args) {
            var sr = new StreamReader();
            //---------------------------------
            var H = sr.Next<int>();
            var W = sr.Next<int>();
            var map = new int[H + 4, W + 4];

            var que = new Queue<int>((H + 4) * (W + 4));

            for(var i = 2; i <= H + 1; i++) {
                var input = sr.Next<string>();
                for(var j = 2; j <= W + 1; j++) {
                    if(input[j - 2] == '#') {
                        map[i, j] = int.MaxValue;
                    } else {
                        que.Enqueue(0);
                        que.Enqueue(j);
                        que.Enqueue(i);
                    }
                }
            }

            for(var i = 2; i <= H + 1; i++) {
                que.Enqueue(0);
                que.Enqueue(1);
                que.Enqueue(i);
                que.Enqueue(0);
                que.Enqueue(W + 2);
                que.Enqueue(i);
            }
            for(var i = 2; i <= W + 1; i++) {
                que.Enqueue(0);
                que.Enqueue(i);
                que.Enqueue(1);
                que.Enqueue(0);
                que.Enqueue(i);
                que.Enqueue(H + 2);
            }


            var dx = new[] { 0, 1, 0, -1, 1, 1, -1, -1 };
            var dy = new[] { 1, 0, -1, 0, 1, -1, 1, -1 };
            var max = 0;

            while(que.Count > 0) {
                var distance = que.Dequeue();
                var x = que.Dequeue();
                var y = que.Dequeue();
                for(var i = 0; i < 8; i++) {
                    var nx = x + dx[i];
                    var ny = y + dy[i];
                    if(map[ny, nx] > distance + 1) {
                        max = Math.Max(max, map[ny, nx] = distance + 1);
                        que.Enqueue(distance + 1);
                        que.Enqueue(nx);
                        que.Enqueue(ny);
                    }
                }
            }

            Console.WriteLine(max);
            //---------------------------------
        }
    }

    public class StreamReader {
        private readonly char[] _c = { ' ' };
        private int _index = -1;
        private string[] _input = new string[0];

        public T Next<T>() {
            if(_index == _input.Length - 1) {
                _index = -1;
                while(true) {
                    string rl = Console.ReadLine();
                    if(rl == null) {
                        if(typeof(T).IsClass) return default(T);
                        return (T)typeof(T).GetField("MinValue").GetValue(null);
                    }
                    if(rl != "") {
                        _input = rl.Split(_c, StringSplitOptions.RemoveEmptyEntries);
                        break;
                    }
                }
            }
            return (T)Convert.ChangeType(_input[++_index], typeof(T), System.Globalization.CultureInfo.InvariantCulture);
        }
    }
}