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

// namespaceの値をコンテスト名にして運用
namespace ABC151D
{
    class Input
    {
        /// <summary>
        /// 1行の入力を取得する
        /// </summary>
        /// <returns>文字列</returns>
        public string String()
        {
            return Console.ReadLine();
        }

        /// <summary>
        /// 複数行の入力を取得
        /// </summary>
        /// <returns>文字列の配列</returns>
        public string[] ArrayString(int rowNumber)
        {
            string[] s = new string[rowNumber];
            for (int i = 0; i < rowNumber; i++)
            {
                s[i] = String();
            }

            return s;
        }

        /// <summary>
        /// 1行の入力を取得
        /// </summary>
        /// <returns>int型の値</returns>
        public int Int()
        {
            return int.Parse(String());
        }

        /// <summary>
        /// 1行の入力を取得
        /// </summary>
        /// <returns>int型の配列</returns>
        public int[] ArrayInt()
        {
            return Split().Select(int.Parse).ToArray();
        }

        /// <summary>
        /// 複数行の入力を取得
        /// </summary>
        /// <param name="rowNumber">行数</param>
        /// <returns>int型の配列</returns>
        public int[] ArrayInt(int rowNumber)
        {
            int[] ints = new int[rowNumber];
            for (int i = 0; i < rowNumber; i++)
            {
                ints[i] = Int();
            }

            return ints;
        }

        /// <summary>
        /// 1行の入力を取得
        /// </summary>
        /// <returns>long型の値</returns>
        public long Long()
        {
            return long.Parse(String());
        }

        /// <summary>
        /// 2つの整数が1行に書かれている入力を、2つのlongで受け取る
        /// </summary>
        /// <param name="A"></param>
        /// <param name="B"></param>
        public void Longs(ref long A, ref long B)
        {
            var longs = ArrayLong();
            A = longs[0];
            B = longs[1];
        }

        /// <summary>
        /// 3つの整数が1行に書かれている入力を、3つのlongで受け取る
        /// </summary>
        /// <param name="A"></param>
        /// <param name="B"></param>
        /// <param name="C"></param>
        public void Longs(ref long A, ref long B, ref long C)
        {
            var longs = ArrayLong();
            A = longs[0];
            B = longs[1];
            C = longs[2];
        }

        /// <summary>
        /// 4つの整数が1行に書かれている入力を、4つのlongで受け取る
        /// </summary>
        /// <param name="A"></param>
        /// <param name="B"></param>
        /// <param name="C"></param>
        /// <param name="D"></param>
        public void Longs(ref long A, ref long B, ref long C, ref long D)
        {
            var longs = ArrayLong();
            A = longs[0];
            B = longs[1];
            C = longs[2];
            D = longs[3];
        }

        /// <summary>
        /// 2つの整数が複数行に書かれている入力を、2つのlong[]で受け取る
        /// </summary>
        /// <param name="rowNumber"></param>
        /// <param name="A"></param>
        /// <param name="B"></param>
        public void LongsArray(long rowNumber, ref long[] A, ref long[] B)
        {
            A = new long[rowNumber];
            B = new long[rowNumber];
            for (int i = 0; i < rowNumber; i++)
            {
                var l = ArrayLong();
                A[i] = l[0];
                B[i] = l[1];
            }
        }

        /// <summary>
        /// 3つの整数が複数行に書かれている入力を、2つのlong[]で受け取る
        /// </summary>
        /// <param name="rowNumber"></param>
        /// <param name="A"></param>
        /// <param name="B"></param>
        /// <param name="C"></param>
        public void LongsArray(long rowNumber, ref long[] A, ref long[] B, ref long[] C)
        {
            A = new long[rowNumber];
            B = new long[rowNumber];
            C = new long[rowNumber];
            for (int i = 0; i < rowNumber; i++)
            {
                var l = ArrayLong();
                A[i] = l[0];
                B[i] = l[1];
                C[i] = l[2];
            }
        }

        /// <summary>
        /// 1行の入力を取得
        /// </summary>
        /// <returns>long型の配列</returns>
        public long[] ArrayLong()
        {
            return Split().Select(long.Parse).ToArray();
        }

        public long[] ArrayLong(int rowNumber)
        {
            long[] longs = new long[rowNumber];
            for (int i = 0; i < rowNumber; i++)
            {
                longs[i] = Long();
            }

            return longs;
        }

        /// <summary>
        /// 1行の入力を取得
        /// </summary>
        /// <returns>double型の値</returns>
        public double Double()
        {
            return double.Parse(String());
        }

        /// <summary>
        /// 1行の入力を取得
        /// </summary>
        /// <returns>double型の配列</returns>
        public double[] ArrayDouble()
        {
            return Split().Select(double.Parse).ToArray();
        }

        private IEnumerable<string> Split()
        {
            return String().Split(' ');
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Solver solver = new Solver();
            solver.Solve();
        }
    }

    class Solver
    {
        public Solver()
        {
            Input input = new Input();
            input.Longs(ref H, ref W);
            S = new bool[H][];
            for (int i = 0; i < H; i++)
            {
                var s = input.String();
                S[i] = new bool[W];
                for (int j = 0; j < s.Length; j++)
                {
                    S[i][j] = s[j] == '.';
                }
            }
        }

        private long H;
        private long W;
        private bool[][] S;

        public void Solve()
        {
            long ans = 0;
            for (int h = 0; h < H; h++)
            {
                for (int w = 0; w < W; w++)
                {
                    if (!S[h][w])
                    {
                        continue;
                    }

                    //探索済み
                    Dictionary<long, long> memo = new Dictionary<long, long>();

                    Queue<long> q = new Queue<long>();
                    q.Enqueue(a(h, w));
                    memo.Add(a(h, w), 0);

                    while (q.Count > 0)
                    {
                        // 今の場所
                        var c = q.Dequeue();

                        List<long> qq = new List<long>();

                        //up
                        if (c - W >= 0)
                        {
                            qq.Add(c - W);
                        }

                        //down
                        if (c + W < H * W)
                        {
                            qq.Add(c + W);
                        }

                        //right
                        if (y(c + 1) == y(c) && c + 1 < H * W)
                        {
                            qq.Add(c + 1);
                        }

                        //left
                        if (y(c - 1) == y(c) && c - 1 >= 0)
                        {
                            qq.Add(c - 1);
                        }

                        foreach (long l in qq)
                        {
                            if (memo.ContainsKey(l))
                            {
                                continue;
                            }

                            if (!isWay(l))
                            {
                                memo.Add(l, -1);
                                continue;
                            }

                            memo.Add(l, memo[c] + 1);
                            q.Enqueue(l);
                        }
                    }

                    long max = 0;

                    foreach (KeyValuePair<long, long> pos in memo)
                    {
                        max = Math.Max(max, pos.Value);
                    }

                    ans = Math.Max(ans, max);
                }
            }

            Console.WriteLine(ans);
        }

        public bool isWay(long a)
        {
            //    Console.WriteLine("" + y(a) + " " + x(a));
            //    Console.WriteLine(S[y(a)][x(a)] ? "way" : "wall");

            return S[y(a)][x(a)];
        }

        public long a(long h, long w)
        {
            return h * W + w;
        }

        public long x(long a)
        {
            return a % W;
        }

        public long y(long a)
        {
            return a / W;
        }
    }
}

Measured memory_bytes: 20568