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

class Program
{
    static void Main(string[] args)
    {
        H = sc.ReadInt();
        W = sc.ReadInt();
        S = sc.ReadStrArray(H);
        //var x = string.Join("\n", S);
        //Console.WriteLine(x.Count(c => c == '.') - 1);
        //H = 20;
        //W = 20;
        //S = new string[20];
        //for (int i = 0; i < 20; i++)
        //{
        //    S[i] = new string('.', 20);
        //}
        var ans = 0;
        for (int sh = 0; sh < H; sh++)
        {
            for (int sw = 0; sw < W; sw++)
            {
                for (int gh = 0; gh < H; gh++)
                {
                    for (int gw = 0; gw < W; gw++)
                    {
                        if (sh == gh && sw == gw) continue;
                        if (NCount(sh, sw) >= 4 || NCount(gh, gw) >= 4) continue;
                        if (S[sh][sw] == '#' || S[gh][gw] == '#') continue;
                        ans = Math.Max(ans, Solve(sh, sw, gh, gw));
                    }
                }
            }
        }
        Console.WriteLine(ans);
    }
    static int NCount(int h, int w)
    {
        var res = 0;
        if (h > 0 && S[h - 1][w] == '.') res++;
        if (w > 0 && S[h][w - 1] == '.') res++;
        if (h < H - 1 && S[h + 1][w] == '.') res++;
        if (w < W - 1 && S[h][w + 1] == '.') res++;
        return res;
    }
    static int Solve(int sh, int sw, int gh, int gw)
    {
        var q = new Queue<Tuple<int, int, int>>();
        q.Enqueue(Tuple.Create(sh, sw, 0));
        var cost = new int[H, W];
        var visited = new bool[H, W];
        visited[sh, sw] = true;
        var res = 0;
        while (q.Count > 0)
        {
            var now = q.Dequeue();
            var h = now.Item1;
            var w = now.Item2;
            var c = now.Item3;
            res = Math.Max(c, res);
            //if (sh == 0 && sw == 0 && gh == 0 && gw == 19) Console.WriteLine(h + " " + w);
            //visited[h, w] = true;
            //cost[h, w] = c;
            if (h == gh && w == gw) break;

            if (h > 0 && !visited[h - 1, w] && S[h - 1][w] == '.')
            {
                visited[h - 1, w] = true;
                cost[h - 1, w] = c + 1;
                q.Enqueue(Tuple.Create(h - 1, w, c + 1));
            }

            if (w > 0 && !visited[h, w - 1] && S[h][w - 1] == '.')
            {
                visited[h, w - 1] = true;
                cost[h, w - 1] = c + 1;
                q.Enqueue(Tuple.Create(h, w - 1, c + 1));
            }

            if (h < H - 1 && !visited[h + 1, w] && S[h + 1][w] == '.')
            {
                visited[h + 1, w] = true;
                cost[h + 1, w] = c + 1;
                q.Enqueue(Tuple.Create(h + 1, w, c + 1));
            }

            if (w < W - 1 && !visited[h, w + 1] && S[h][w + 1] == '.')
            {
                visited[h, w + 1] = true;
                cost[h, w + 1] = c + 1;
                q.Enqueue(Tuple.Create(h, w + 1, c + 1));
            }
        }
        //return cost[gh, gw];
        return res;
    }
    static int H;
    static int W;
    static string[] S;
    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: 17600