using System;

public class Program
{
    private int H, W;
    private int[,] Edge;

    private char[][] S;
    private const int Inf = 100000;

    public void Solve()
    {
        var sc = new Scanner();
        H = sc.NextInt();
        W = sc.NextInt();
        int HW = H * W;
        Edge = new int[HW, HW];

        for (int i = 0; i < HW; i++)
        {
            for (int j = 0; j < HW; j++)
            {
                Edge[i, j] = Inf;
            }
        }

        S = new char[H][];
        for (int i = 0; i < H; i++)
        {
            S[i] = sc.Next().ToCharArray();
        }

        for (int i = 0; i < H; i++)
        {
            for (int j = 0; j < W; j++)
            {
                if (S[i][j] == '#') continue;
                int c = i * W + j;
                if (i - 1 >= 0 && S[i - 1][j] == '.')
                {
                    int p = (i - 1) * W + j;
                    Edge[c, p] = 1;
                    Edge[p, c] = 1;
                }

                if (j - 1 >= 0 && S[i][j - 1] == '.')
                {
                    int p = i * W + j - 1;
                    Edge[c, p] = 1;
                    Edge[p, c] = 1;
                }

                if (i + 1 < H && S[i + 1][j] == '.')
                {
                    int p = (i + 1) * W + j;
                    Edge[c, p] = 1;
                    Edge[p, c] = 1;
                }

                if (j + 1 < W && S[i][j + 1] == '.')
                {
                    int p = i * W + j + 1;
                    Edge[c, p] = 1;
                    Edge[p, c] = 1;
                }
            }
        }

        for (int i = 0; i < HW; i++)
        {
            for (int j = 0; j < HW; j++)
            {
                for (int k = 0; k < HW; k++)
                {
                    Edge[j, k] = Math.Min(Edge[j, k], Edge[j, i] + Edge[i, k]);
                }
            }
        }

        int max = -1;
        for (int i = 0; i < HW; i++)
        {
            for (int j = 0; j < HW; j++)
            {
                if (Edge[i, j] == Inf)
                    continue;
                if (i == j) continue;
                max = Math.Max(max, Edge[i, j]);
            }
        }

        Console.WriteLine(max);
    }

    public static void Main(string[] args)
    {
        new Program().Solve();
    }
}

class Scanner
{
    public Scanner()
    {
        _pos = 0;
        _line = new string[0];
    }

    const char Separator = ' ';
    private int _pos;
    private string[] _line;

    #region スペース区切りで取得

    public string Next()
    {
        if (_pos >= _line.Length)
        {
            _line = Console.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 型変換

    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 配列取得

    public string[] Array()
    {
        if (_pos >= _line.Length)
            _line = Console.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: 13276