using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace AtCoder
{
    partial class Program
    {
        static void Main()
        {
            Console.SetOut(new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false });
            Solve();
            Console.Out.Flush();
            Console.ReadKey();
        }

        static int H;
        static int W;
        static char[][] S;
        static int[][] len;

        static void Solve()
        {
            Multi(out H, out W);
            S = new char[H][].Select(c => GetStr().ToCharArray()).ToArray();
            var ans = 0;
            for (int i = 0; i < H; i++)
            {
                for(int j = 0; j < W; j++)
                {
                    len = Enumerable.Repeat(0, H).Select(s => Enumerable.Repeat(-1, W).ToArray()).ToArray();
                    if (S[i][j] == '#') continue;
                    len[i][j] = 0;
                    DFS(i,j);
                    ans = Math.Max(ans, len.Select(l=>l.Max()).Max());
                }
            }

            Console.WriteLine(ans);
        }

        static Queue<Tuple<int,int>> next = new Queue<Tuple<int,int>>();
        static void DFS(int i,int j)
        {
            var vec = new int[,] { { 1, 0 }, { -1, 0 }, { 0, 1 }, { 0, -1 } };
            for(int v = 0; v < 4; v++)
            {
                var ii = i + vec[v, 0];
                var jj = j + vec[v, 1];
                if (ii < 0 || H - 1 < ii || jj < 0 || W - 1 < jj) continue;
                if (len[ii][jj] != -1) continue;
                if (S[ii][jj] == '#') continue;
                len[ii][jj] = len[i][j] + 1;
                next.Enqueue(new Tuple<int, int>(ii, jj));
            }
            while (next.Count() > 0)
            {
                var n = next.Dequeue();
                DFS(n.Item1, n.Item2);
            }
        }
    }

    //拡張メソッド
    public static class Ex
    {
        public static List<string> FastSort(this List<string> s) { s.Sort(StringComparer.Ordinal); return s.ToList(); }
        public static string yesno(this bool b) { return b ? "yes" : "no"; }
        public static string YesNo(this bool b) { return b ? "Yes" : "No"; }
        public static string YESNO(this bool b) { return b ? "YES" : "NO"; }
    }

    partial class Program
    {
        static public string GetStr() { return Console.ReadLine().Trim(); }
        static public char GetChar() { return Console.ReadLine().Trim()[0]; }
        static public int GetInt() { return int.Parse(Console.ReadLine().Trim()); }
        static public long GetLong() { return long.Parse(Console.ReadLine().Trim()); }
        static public double GetDouble() { return double.Parse(Console.ReadLine().Trim()); }
        static public string[] GetStrArray() { return Console.ReadLine().Trim().Split(' '); }
        static public int[] GetIntArray() { return Console.ReadLine().Trim().Split(' ').Select(int.Parse).ToArray(); }
        static public long[] GetLongArray() { return Console.ReadLine().Trim().Split(' ').Select(long.Parse).ToArray(); }
        static public char[] GetCharArray() { return Console.ReadLine().Trim().Split(' ').Select(char.Parse).ToArray(); }
        static public double[] GetDoubleArray() { return Console.ReadLine().Trim().Split(' ').Select(double.Parse).ToArray(); }
        static public void WriteObjects<T>(IList<T> values) { var num = values.Count; for (int i = 0; i < num; i++) { Console.Write(values[i]); if (i < num - 1) { Console.Write(" "); } } Console.WriteLine(); }

        static bool eq<T, U>() => typeof(T).Equals(typeof(U));
        static T ct<T, U>(U a) => (T)Convert.ChangeType(a, typeof(T));
        static T cv<T>(string s) => eq<T, int>() ? ct<T, int>(int.Parse(s))
                           : eq<T, long>() ? ct<T, long>(long.Parse(s))
                           : eq<T, double>() ? ct<T, double>(double.Parse(s))
                           : eq<T, char>() ? ct<T, char>(s[0])
                                             : ct<T, string>(s);
        static void Multi<T>(out T a) => a = cv<T>(GetStr());
        static void Multi<T, U>(out T a, out U b)
        {
            var ar = GetStrArray(); a = cv<T>(ar[0]); b = cv<U>(ar[1]);
        }
        static void Multi<T, U, V>(out T a, out U b, out V c)
        {
            var ar = GetStrArray(); a = cv<T>(ar[0]); b = cv<U>(ar[1]); c = cv<V>(ar[2]);
        }
        static void Multi<T, U, V, W>(out T a, out U b, out V c, out W d)
        {
            var ar = GetStrArray(); a = cv<T>(ar[0]); b = cv<U>(ar[1]); c = cv<V>(ar[2]); d = cv<W>(ar[3]);
        }
        static void Multi<T, U, V, W, X>(out T a, out U b, out V c, out W d, out X e)
        {
            var ar = GetStrArray(); a = cv<T>(ar[0]); b = cv<U>(ar[1]); c = cv<V>(ar[2]); d = cv<W>(ar[3]); e = cv<X>(ar[4]);
        }
    }
}

Measured memory_bytes: 18756