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

class Program
{
    static void Main(string[] args)
    {
        var H = sc.ReadInt();
        var W = sc.ReadInt();
        var S = sc.ReadStrArray(H);
        Func<int, int, int> to1d = (h, w) => W * h + w;
        var dh = new int[] { -1, 0, 1, 0 };
        var dw = new int[] { 0, -1, 0, 1 };
        var dp = new int[H * W, H * W];
        for (int i = 0; i < H * W; i++)
        {
            for (int j = 0; j < H * W; j++)
            {
                dp[i, j] = 10000;
            }
        }
        for (int i = 0; i < H * W; i++)
        {
            var h = i / W;
            var w = i % W;
            if (S[h][w] == '#') continue;
            dp[i, i] = 0;
            for (int id = 0; id < 4; id++)
            {
                var nh = h + dh[id];
                var nw = w + dw[id];
                if (0 <= nh && nh < H && 0 <= nw && nw < W && S[nh][nw] != '#')
                {
                    dp[to1d(h, w), to1d(nh, nw)] = 1;
                }
            }
        }
        for (int k = 0; k < H * W; k++)
        {
            var hk = k / W;
            var wk = k % W;
            if (S[hk][wk] == '#') continue;
            for (int i = 0; i < H * W; i++)
            {
                var hi = i / W;
                var wi = i % W;
                if (S[hi][wi] == '#') continue;
                for (int j = 0; j < H * W; j++)
                {
                    var hj = j / W;
                    var wj = j % W;
                    if (S[hj][wj] == '#') continue;
                    var a = to1d(hi, wi);
                    var b = to1d(hj, wj);
                    var c = to1d(hk, wk);
                    if (a == 1 && b == 1 && c == 0)
                    {

                    }
                    dp[a, b] = Math.Min(dp[a, b], dp[a, c] + dp[c, b]);
                }
            }
        }

        var ans = 0;
        for (int i = 0; i < H * W; i++)
        {
            for (int j = 0; j < H * W; j++)
            {
                if (dp[i, j] >= 10000) continue;
                ans = Math.Max(ans, dp[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: 13140