Problem:
Score : 400 points

Problem Statement

Takahashi has a maze, which is a grid of H \times W squares with H horizontal rows and W vertical columns.

The square at the i-th row from the top and the j-th column is a "wall" square if S_{ij} is #, and a "road" square if S_{ij} is ..

From a road square, you can move to a horizontally or vertically adjacent road square.

You cannot move out of the maze, move to a wall square, or move diagonally.

Takahashi will choose a starting square and a goal square, which can be any road squares, and give the maze to Aoki.

Aoki will then travel from the starting square to the goal square, in the minimum number of moves required.

In this situation, find the maximum possible number of moves Aoki has to make.

Constraints

1 \leq H,W \leq 20

S_{ij} is . or #.

S contains at least two occurrences of ..

Any road square can be reached from any road square in zero or more moves.

Input

Input is given from Standard Input in the following format:

H W
S_{11}...S_{1W}
:
S_{H1}...S_{HW}

Output

Print the maximum possible number of moves Aoki has to make.

Sample Input 1

3 3
...
...
...

Sample Output 1

4

If Takahashi chooses the top-left square as the starting square and the bottom-right square as the goal square, Aoki has to make four moves.

Sample Input 2

3 5
...#.
.#.#.
.#...

Sample Output 2

10

If Takahashi chooses the bottom-left square as the starting square and the top-right square as the goal square, Aoki has to make ten moves.

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Reflection;
using System.Text;

namespace A
{
    class StartingPoint
    {
        static void Main(string[] args)
        {
            if (args?.Any() == true)
            {
                var assembly = Assembly.GetEntryAssembly();
                var resourceName = "AtCoderCSharp.In.txt";
                var stream = assembly.GetManifestResourceStream(resourceName);
                var streamReader = new StreamReader(stream, Encoding.UTF8);
                var str = streamReader.ReadToEnd();
                var textReader = new StringReader(str);
                Console.SetIn(textReader);
            }

            try
            {
                foreach (var s in new Program().Solve())
                {
                    Console.WriteLine(s);
                }
            }
            catch (Exception e)
            {
                Console.WriteLine(e);
                if (args?.Any() != true)
                    throw e;
            }

            if (args?.Any() == true)
                Console.ReadKey();
        }
    }

    class Program
    {
        public List<int> ScanInts(int count) => Enumerable.Range(0, count).Select(_ => ScanInt).ToList();
        public List<int> ScanInts(long count) => ScanInts((int)count);
        public List<long> ScanLongs(int count) => Enumerable.Range(0, count).Select(_ => Scan).ToList();
        public List<long> ScanLongs(long count) => ScanLongs((int)count);
        public int ScanInt => int.Parse(Str);
        public long Scan => long.Parse(Str);
        public double ScanDouble => (double)Scan;

        StringBuilder sb = new StringBuilder();

        public string Str => ScanStr();
        public string ScanStr()
        {
            sb.Clear();

            int r = 0;
            bool? b = null;
            do
            {
                r = Console.Read();
                b = IsBreak(r);
            } while (b == true);

            if (b == null)
                throw new Exception("input error: unexpected end of stream");

            do
            {
                sb.Append((char)r);
                r = Console.Read();
                b = IsBreak(r);
            } while (b == false);

            return sb.ToString();
        }

        bool? IsBreak(int c)
        {
            switch (c)
            {
                case -1:
                    return null;
                case (int)' ':
                case (int)'\n':
                case (int)'\r':
                    return true;
                default:
                    return false;
            }
        }

        public IEnumerable<int> Loop(int count) => Enumerable.Range(0, count);
        public IEnumerable<int> Loop(int from, int to) => Enumerable.Range(from, to - from + 1);

        public IEnumerable<long> Loop(long count) => Loop(0, count - 1);
        public IEnumerable<long> Loop(long from, long to)
        {
            for (long i = from; i <= to; i++)
                yield return i;
        }

        public string YesNo(bool isYes) => isYes ? "Yes" : "No";

        public HashSet<T> ToHashSet<T>(IEnumerable<T> ts) => new HashSet<T>(ts);

        public IEnumerable<string> Solve()
        {
            var h = ScanInt;
            var w = ScanInt;
            var s = new List<string>();

            s.Add(new string(Enumerable.Range(0, w + 2).Select(_ => '#').ToArray()));

            foreach (var i in Loop(h))
            {
                var ss = "#" + ScanStr() + "#";
                s.Add(ss);
            }

            s.Add(new string(Enumerable.Range(0, w + 2).Select(_ => '#').ToArray()));


            var res = 0;

            foreach (var i in Loop(h + 2).Skip(1).Take(h))
            {
                foreach (var j in Loop(w + 2).Skip(1).Take(w))
                {
                    if (s[i][j] != '.')
                        continue;

                    var c = Enumerable.Range(0, h + 2).Select(x => Enumerable.Range(0, w + 2).Select(y => -1).ToArray()).ToArray();
                    var p = Enumerable.Range(0, h + 2).Select(x => Enumerable.Range(0, w + 2).Select(y => false).ToArray()).ToArray();

                    c[i][j] = 0;

                    var checking = new HashSet<int>();
                    checking.Add(i + j * 25);

                    while (checking.Any())
                    {
                        var newChecking = new HashSet<int>();

                        foreach (var xy in checking.ToArray())
                        {
                            var x = xy % 25;
                            var y = xy / 25;

                            p[x][y] = true;

                            for (int z = 0; z < 4; z++)
                            {
                                var xx = x;
                                var yy = y;
                                switch (z)
                                {
                                    case 0: xx--; break;
                                    case 1: yy--; break;
                                    case 2: xx++; break;
                                    case 3: yy++; break;
                                }

                                if (s[xx][yy] == '.')
                                {
                                    if (c[xx][yy] < 0)
                                        c[xx][yy] = c[x][y] + 1;
                                    else
                                        c[xx][yy] = Math.Min(c[xx][yy], c[x][y] + 1);

                                    var xxyy = xx + yy * 25;
                                    if (p[xx][yy] == false && checking.Contains(xxyy) == false && newChecking.Contains(xxyy) == false)
                                        newChecking.Add(xxyy);

                                }
                            }
                        }

                        checking = newChecking;
                    }

                    var cand = c.SelectMany(x => x).Max();
                    res = Math.Max(cand, res);
                }
            }


            yield return res.ToString();
        }
    }
}