using System;
using System.Threading.Tasks;

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[5, H + 2, W + 2];

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

            Parallel.Invoke(
                () =>{
                    for(var i = 1; i <= H; i++){
                        for(var j = 1; j <= W; j++){
                            map[1, i, j] =
                                Min4(map[0, i, j], map[1, i - 1, j] + 1, map[1, i, j - 1] + 1, map[1, i - 1, j - 1] + 1);
                        }
                    }
                },
                () =>{
                    for(var i = H; i >= 1; i--){
                        for(var j = 1; j <= W; j++){
                            map[2, i, j] =
                                Min4(map[0, i, j], map[2, i + 1, j] + 1, map[2, i, j - 1] + 1, map[2, i + 1, j - 1] + 1);
                        }
                    }
                },
                () =>{
                    for(var i = 1; i <= H; i++){
                        for(var j = W; j >= 1; j--){
                            map[3, i, j] =
                                Min4(map[0, i, j], map[3, i - 1, j] + 1, map[3, i, j + 1] + 1, map[3, i - 1, j + 1] + 1);
                        }
                    }
                }, () =>{
                    for(var i = H; i >= 1; i--){
                        for(var j = W; j >= 1; j--){
                            map[4, i, j] =
                                Min4(map[0, i, j], map[4, i + 1, j] + 1, map[4, i, j + 1] + 1, map[4, i + 1, j + 1] + 1);
                        }
                    }
                });

            var max = 0;
            for(var i = 1; i <= H; i++){
                for(var j = 1; j <= W; j++){
                    max = Math.Max(max, Min4(map[1, i, j], map[2, i, j], map[3, i, j], map[4, i, j]));
                }
            }

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

        public static int Min4(int a1, int a2, int a3, int a4){
            return Math.Min(a1, Math.Min(a2, Math.Min(a3, a4)));
        }
        //---------------------------------
    }

    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);
        }

        public T[] Next<T>(int x){
            var ret = new T[x];
            for(var i = 0; i < x; ++i) ret[i] = Next<T>();
            return ret;
        }
    }
}