using System;

namespace YukiMe2_2 {
    public class Program {
        public static void Main(string[] args) {
            var sr = new StreamReader();
            //---------------------------------
            var H = sr.Next<int>();
            var W = sr.Next<int>();
            var M = Math.Min(H, W) / 2;
            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;
                }
            }

            Func<int, int, bool> isInside = (x, y) => 0 <= x && x < H + 2 && 0 <= y && y < W + 2;

            var dx = new[]{0, 1, 0, -1, 1, 1, -1, -1};
            var dy = new[]{1, 0, -1, 0, -1, 1, 1, -1};
            for(var k = 0; k <= M; k++){
                for(var i = 0; i < H + 2; i++){
                    for(var j = 0; j < W + 2; j++){
                        if(map[i, j] == k){
                            for(var d = 0; d < 8; d++){
                                if(isInside(j + dx[d], i + dy[d])){
                                    map[i + dy[d], j + dx[d]] = Math.Min(map[i + dy[d], j + dx[d]], k + 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);
        }
    }
}