using System;
using System.Collections.Generic;

namespace YukiMe2_6{
    public class Program{
        public static void Main(string[] args){
            var sr = new StreamReader();
            //---------------------------------
            var H = sr.Next<int>();
            var W = sr.Next<int>();
            var dist = new int[H + 4, W + 4];
            var que = new Queue<Tuple<int, int, int>>();

            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] == '#'){
                        dist[i, j] = 1510;
                    } else{
                        que.Enqueue(Tuple.Create(0, j, i));
                    }
                }
            }

            for(var i = 2; i <= H + 1; i++){
                que.Enqueue(Tuple.Create(0, 1, i));
                que.Enqueue(Tuple.Create(0, W + 2, i));
            }
            for(var i = 2; i <= W + 1; i++){
                que.Enqueue(Tuple.Create(0, i, 1));
                que.Enqueue(Tuple.Create(0, i, 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 deq = que.Dequeue();
                var t = deq.Item1;
                var x = deq.Item2;
                var y = deq.Item3;
                for(var i = 0; i < 8; i++){
                    var nx = x + dx[i];
                    var ny = y + dy[i];
                    if(dist[ny, nx] > t + 1){
                        max = Math.Max(max, dist[ny, nx] = t + 1);
                        que.Enqueue(Tuple.Create(t + 1, nx, 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);
        }
    }
}