using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using static System.Math;
using System.Text;
using System.Threading;
using System.Globalization;
using System.Runtime.CompilerServices;
using Library;

namespace Program
{
    public static class ABC151D
    {
        static public int numberOfRandomCases = 0;
        static public void MakeTestCase(List<string> _input, List<string> _output, ref Func<string[], bool> _outputChecker)
        {
        }
        static public void Solve()
        {
            var H = NN;
            var W = NN;
            var S = NSList(H);
            var maxCnt = 0L;
            Func<long, long, long, long> check = null;
            var nagasa = new long[H, W];
            check = (sx, sy, len) =>
            {
                if (sx < 0 || sx >= H || sy < 0 || sy >= W) return 0;
                if (S[sx][(int)sy] == '#') return 0;
                if (nagasa[sx, sy] <= len) return 0;
                nagasa[sx, sy] = len;
                check(sx + 1, sy, len + 1);
                check(sx, sy + 1, len + 1);
                check(sx - 1, sy, len + 1);
                check(sx, sy - 1, len + 1);
                return 0;
            };
            for (var sx = 0; sx < H; sx++)
            {
                for (var sy = 0; sy < W; sy++)
                {
                    nagasa = new long[H, W];
                    for (var gx = 0; gx < H; gx++)
                    {
                        for (var gy = 0; gy < W; gy++)
                        {
                            nagasa[gx, gy] = 10000000000;
                        }
                    }
                    check(sx, sy, 0);
                    for (var gx = 0; gx < H; gx++)
                    {
                        for (var gy = 0; gy < W; gy++)
                        {
                            if (nagasa[gx, gy] < 1000000000) maxCnt = Max(maxCnt, nagasa[gx, gy]);
                        }
                    }
                }
            }

            Console.WriteLine(maxCnt);
        }
        static class Console_
        {
            static Queue<string> param = new Queue<string>();
            public static string NextString() { if (param.Count == 0) foreach (var item in Console.ReadLine().Split(' ')) param.Enqueue(item); return param.Dequeue(); }
        }
        class Printer : StreamWriter
        {
            public override IFormatProvider FormatProvider { get { return CultureInfo.InvariantCulture; } }
            public Printer(Stream stream) : base(stream, new UTF8Encoding(false, true)) { base.AutoFlush = false; }
            public Printer(Stream stream, Encoding encoding) : base(stream, encoding) { base.AutoFlush = false; }
        }
        static public void Main(string[] args) { if (args.Length == 0) { Console.SetOut(new Printer(Console.OpenStandardOutput())); } var t = new Thread(Solve, 134217728); t.Start(); t.Join(); Console.Out.Flush(); }
        static long NN => long.Parse(Console_.NextString());
        static double ND => double.Parse(Console_.NextString());
        static string NS => Console_.NextString();
        static long[] NNList(long N) => Repeat(0, N).Select(_ => NN).ToArray();
        static double[] NDList(long N) => Repeat(0, N).Select(_ => ND).ToArray();
        static string[] NSList(long N) => Repeat(0, N).Select(_ => NS).ToArray();
        static IEnumerable<T> OrderByRand<T>(this IEnumerable<T> x) => x.OrderBy(_ => xorshift);
        static long Count<T>(this IEnumerable<T> x, Func<T, bool> pred) => Enumerable.Count(x, pred);
        static IEnumerable<T> Repeat<T>(T v, long n) => Enumerable.Repeat<T>(v, (int)n);
        static IEnumerable<int> Range(long s, long c) => Enumerable.Range((int)s, (int)c);
        static uint xorshift { get { _xsi.MoveNext(); return _xsi.Current; } }
        static IEnumerator<uint> _xsi = _xsc();
        static IEnumerator<uint> _xsc() { uint x = 123456789, y = 362436069, z = 521288629, w = (uint)(DateTime.Now.Ticks & 0xffffffff); while (true) { var t = x ^ (x << 11); x = y; y = z; z = w; w = (w ^ (w >> 19)) ^ (t ^ (t >> 8)); yield return w; } }
    }
}
namespace Library {
}


Measured memory_bytes: 14944