using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

class Program
{
    static void Main(string[] args)
    {
        var H = sc.ReadInt();
        var W = sc.ReadInt();
        var a = new bool[H, W];
        for (int i = 0; i < H; i++)
        {
            var S = sc.ReadStr();
            for (int j = 0; j < W; j++)
            {
                if (S[j] == '#')
                {
                    a[i, j] = true;
                }
            }
        }

        var count = new int[H, W];
        for (int i = 0; i < H; i++)
        {
            for (int j = 0; j < W; j++)
            {
                // 大きい数字にしておく
                count[i, j] = 10000001;
                if (a[i, j]) count[i, j] = 0;
            }
        }
        // 右に見ていく
        for (int i = 0; i < H; i++)
        {
            for (int j = 1; j < W; j++)
            {
                count[i, j] = Math.Min(count[i, j], count[i, j - 1] + 1);
            }
        }
        // 左に見ていく
        for (int i = 0; i < H; i++)
        {
            for (int j = W - 2; j >= 0; j--)
            {
                count[i, j] = Math.Min(count[i, j], count[i, j + 1] + 1);
            }
        }
        // 下に見ていく
        for (int j = 0; j < W; j++)
        {
            for (int i = 1; i < H; i++)
            {
                count[i, j] = Math.Min(count[i, j], count[i - 1, j] + 1);
            }
        }
        // 上に見ていく
        for (int j = 0; j < W; j++)
        {
            for (int i = H - 2; i >= 0; i--)
            {
                count[i, j] = Math.Min(count[i, j], count[i + 1, j] + 1);
            }
        }

        var ans = int.MinValue;
        for (int i = 0; i < H; i++)
        {
            for (int j = 0; j < W; j++)
            {
                ans = Math.Max(ans, count[i, j]);
            }
        }
        Console.WriteLine(ans);
    }
    static Scanner sc = new Scanner();
}

class Scanner
{
    string[] S = new string[0];
    int Index = 0;
    char[] Separators = new char[] { ' ' };

    public string Next()
    {
        if (this.Index < this.S.Length) return this.S[this.Index++];
        var line = "";
        while (line == "") line = Console.ReadLine();
        this.S = line.Split(this.Separators, StringSplitOptions.RemoveEmptyEntries);
        if (this.S.Length == 0) return this.Next();
        this.Index = 0;
        return this.S[this.Index++];
    }
    public string ReadStr()
    {
        return this.Next();
    }
    public char ReadChar()
    {
        return this.Next()[0];
    }
    public int ReadInt()
    {
        return int.Parse(this.Next());
    }
    public uint ReadUInt()
    {
        return uint.Parse(this.Next());
    }
    public long ReadLong()
    {
        return long.Parse(this.Next());
    }
    public double ReadDouble()
    {
        return double.Parse(this.Next());
    }
    public Tuple<int, int> ReadTup(int add = 0)
    {
        return Tuple.Create(this.ReadInt() + add, this.ReadInt() + add);
    }
    public Tuple<long, long> ReadTupLong(int add = 0)
    {
        return Tuple.Create(this.ReadLong() + add, this.ReadLong() + add);
    }
    public Tuple<int, int, int> ReadTup3(int add = 0)
    {
        return Tuple.Create(this.ReadInt() + add, this.ReadInt() + add, this.ReadInt() + add);
    }
    public Tuple<int, int, int, int> ReadTup4(int add = 0)
    {
        return Tuple.Create(this.ReadInt() + add, this.ReadInt() + add, this.ReadInt() + add, this.ReadInt() + add);
    }
    public int[] ReadIntArray(int n)
    {
        var array = new int[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadInt();
        }
        return array;
    }
    public long[] ReadLongArray(int n)
    {
        var array = new long[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadLong();
        }
        return array;
    }
    public double[] ReadDoubleArray(int n)
    {
        var array = new double[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadDouble();
        }
        return array;
    }
    public char[] ReadCharArray(int n)
    {
        var array = new char[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadChar();
        }
        return array;
    }
    public string[] ReadStrArray(int n)
    {
        var array = new string[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadStr();
        }
        return array;
    }
    public Tuple<long, long>[] ReadTupLongArray(int n, int add = 0)
    {
        var array = new Tuple<long, long>[n];
        for (int i = 0; i < n; i++)
        {
            array[i] = this.ReadTupLong(add);
        }
        return array;
    }
    public Tuple<int, int>[] ReadTupArray(int n, int add = 0)
    {
        var array = new Tuple<int, int>[n];
        for (int i = 0; i < n; i++)
        {
            array[i] = this.ReadTup(add);
        }
        return array;
    }
    public Tuple<int, int, int>[] ReadTup3Array(int n, int add = 0)
    {
        var array = new Tuple<int, int, int>[n];
        for (int i = 0; i < n; i++)
        {
            array[i] = this.ReadTup3(add);
        }
        return array;
    }
    public Tuple<int, int, int, int>[] ReadTup4Array(int n, int add = 0)
    {
        var array = new Tuple<int, int, int, int>[n];
        for (int i = 0; i < n; i++)
        {
            array[i] = this.ReadTup4(add);
        }
        return array;
    }
}


Measured memory_bytes: 21976