using System;

namespace YukiMe2_3 {
    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 + 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[i, j] = int.MaxValue;
                }
            }

            for(var i = 1; i <= H; i++) {
                for(var j = 1; j <= W; j++) {
                    map[i, j] = Min4(map[i, j], map[i - 1, j] + 1, map[i, j - 1] + 1, map[i - 1, j - 1] + 1);
                }
            }
            for(var i = H; i >= 1; i--) {
                for(var j = 1; j <= W; j++) {
                    map[i, j] = Min4(map[i, j], map[i + 1, j] + 1, map[i, j - 1] + 1, map[i + 1, j - 1] + 1);
                }
            }
            for(var i = 1; i <= H; i++) {
                for(var j = W; j >= 1; j--) {
                    map[i, j] = Min4(map[i, j], map[i - 1, j] + 1, map[i, j + 1] + 1, map[i - 1, j + 1] + 1);
                }
            }
            for(var i = H; i >= 1; i--) {
                for(var j = W; j >= 1; j--) {
                    map[i, j] = Min4(map[i, j], map[i + 1, j] + 1, map[i, j + 1] + 1, map[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, map[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);
        }
    }
}