using System;
using System.Collections;
using System.Collections.Generic;
using System.IO;
using System.Linq;

namespace Contest
{
    class Program
    {
        private int H, W;
        private char[][] A;
        private int[] nextY = new int[] { 1, 0, -1, 0 };
        private int[] nextX = new int[] { 0, 1, 0, -1 };
        public void Solve()
        {
            var sc = new Scanner();
            H = sc.NextInt();
            W = sc.NextInt();
            A = new char[H][];
            for (int i = 0; i < H; i++)
            {
                A[i] = sc.Next().ToArray();
            }
            var hq = new Queue<int>();
            var wq = new Queue<int>();
            var cnt = new int[H, W];
            for (int i = 0; i < H; i++)
            {
                for (int j = 0; j < W; j++)
                {
                    cnt[i, j] = -1;
                    if (A[i][j] == '#')
                    {
                        cnt[i, j] = 0;
                        hq.Enqueue(i);
                        wq.Enqueue(j);
                    }
                }
            }

            while (hq.Count > 0)
            {
                int h = hq.Dequeue();
                int w = wq.Dequeue();
                for (int i = 0; i < 4; i++)
                {
                    int nY = h + nextY[i];
                    int nX = w + nextX[i];
                    if (nY < 0 || nY >= H) continue;
                    if (nX < 0 || nX >= W) continue;
                    if(cnt[nY,nX] != -1) continue;
                    cnt[nY, nX] = cnt[h, w] + 1;
                    hq.Enqueue(nY);
                    wq.Enqueue(nX);
                }

            }

            int ans = 0;
            for (int i = 0; i < H; i++)
            {
                for (int j = 0; j < W; j++)
                {
                    ans = Math.Max(ans, cnt[i, j]);
                }
            }
            Console.WriteLine(ans);
        }

        static void Main() => new Program().Solve();
    }
}

class Scanner
{
    public Scanner()
    {
        _stream = new StreamReader(Console.OpenStandardInput());
        _pos = 0;
        _line = new string[0];
        _separator = ' ';
    }
    private char _separator;
    private StreamReader _stream;
    private int _pos;
    private string[] _line;
    #region get a element
    public string Next()
    {
        if (_pos >= _line.Length)
        {
            _line = _stream.ReadLine().Split(_separator);
            _pos = 0;
        }
        return _line[_pos++];
    }
    public int NextInt()
    {
        return int.Parse(Next());
    }
    public long NextLong()
    {
        return long.Parse(Next());
    }
    public double NextDouble()
    {
        return double.Parse(Next());
    }
    #endregion
    #region convert array
    private int[] ToIntArray(string[] array)
    {
        var result = new int[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = int.Parse(array[i]);
        }

        return result;
    }
    private long[] ToLongArray(string[] array)
    {
        var result = new long[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = long.Parse(array[i]);
        }

        return result;
    }
    private double[] ToDoubleArray(string[] array)
    {
        var result = new double[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = double.Parse(array[i]);
        }

        return result;
    }
    #endregion
    #region get array
    public string[] Array()
    {
        if (_pos >= _line.Length)
            _line = _stream.ReadLine().Split(_separator);
        _pos = _line.Length;
        return _line;
    }
    public int[] IntArray()
    {
        return ToIntArray(Array());
    }
    public long[] LongArray()
    {
        return ToLongArray(Array());
    }
    public double[] DoubleArray()
    {
        return ToDoubleArray(Array());
    }
    #endregion
}

Measured memory_bytes: 32036