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.Linq;
using System.Text;

// using System.Numerics;
using System.Runtime.CompilerServices;
using System.Diagnostics;
using ll=System.Int64;
using static Contest_D.MyIO;



public class Contest_D
{
	static void swap1<T>(ref T a, ref T b) { T t = a; a = b; b = t; }
	static void swap2<T>(ref T a, ref T b) where T : IComparable { if (a.CompareTo(b)==1) swap1(ref a, ref b); }  // b の方が小さければ交換
	static bool chmin<T>(ref T a, T b) where T : IComparable { if (a.CompareTo(b)== 1) { a = b; return true; } return false; }  // b の方が小さければ a を更新
	static bool chmax<T>(ref T a, T b) where T : IComparable { if (a.CompareTo(b)==-1) { a = b; return true; } return false; }  // b の方が大きければ a を更新
	static bool inside(long i, long n) => (0<=i&&i<n);
	static bool inside(long x, long y, long w, long h) => (inside(x,w)&&inside(y,h));
	static T min<T>(params T[] a) where T : IComparable => a.Min();
	static T max<T>(params T[] a) where T : IComparable => a.Max();
	static long mod(long a, long m) { var v = a%m; return (v<0?v+m:v); }
	static T[][] init<T>(long len1,long len2,T val) where T : struct { T[][] m = new T[len1][]; for (int i=0;i<m.Length;i++) m[i] = Enumerable.Repeat(val,(int)len2).ToArray(); return m; }
	static T[][][] init<T>(long len1,long len2,long len3,T val) where T : struct { T[][][] m = new T[len1][][]; for (int i=0;i<m.Length;i++) m[i] = init(len2,len3,val); return m; }
	public const long MOD1 = 1000000007; // 10^9+7
	public const double EPS1 = 1e-8;
	public const long INF1 = 1000000000000000;  // 10^15

	public static void Main() {
		long  h, w;
		MyIO.RM(out h, out w);
		string[] s;
		RL(h,out s);

		ll ans = 0;
		ll[][] gl = new ll[h*w][];
		for (int sx = 0; sx < h; sx++) for (int sy = 0; sy < w; sy++) {
			var l = new List<ll>();
			if(s[sx][sy]=='.') {
				if(sx<h-1&&s[sx+1][sy]=='.') l.Add((sx+1)*w+sy);
				if(0<sx&&s[sx-1][sy]=='.') l.Add((sx-1)*w+sy);
				if(sy<w-1&&s[sx][sy+1]=='.') l.Add(sx*w+sy+1);
				if(0<sy&&s[sx][sy-1]=='.') l.Add(sx*w+sy-1);
			}
			gl[sx*w+sy] = l.ToArray();
		}
		for (int sx = 0; sx < h; sx++) for (int sy = 0; sy < w; sy++) {
			if(s[sx][sy]=='.') chmax(ref ans, Search(h*w,sx*w+sy, gl));
		}

		MyIO.WM(ans);
	}


	public class ModQue<T>{
		public Action Clear;
		public Action<T> In;
		public Func<T> Out, Peek;
		public Func<int> Count;
		public ModQue(int type){
			if(type==1) {
				var S = new Stack<T>();
				Clear = S.Clear;
				In = S.Push;
				Out = S.Pop;
				Peek = S.Peek;
				Count = () => S.Count;
			}
			if(type==2) {
				var Q = new Queue<T>();
				Clear = Q.Clear;
				In = Q.Enqueue;
				Out = Q.Dequeue;
				Peek = Q.Peek;
				Count = () => Q.Count;
			}
		}
	}

    public static long Search(long N, long start, IList<IList<long>> map){
		long count = 0;
        int stype = 2;  // 1:DFS 2:BFS
        ModQue<VT<long,int>> que = new ModQue<VT<long,int>>(stype);
        que.In(new VT<long, int>(start,0));
        bool[] visited = new bool[N]; 
        while(0<que.Count()){
            VT<long,int> now = que.Out();
            if(visited[now.Item1]) continue;
            visited[now.Item1]=true;
			int nc = now.Item2;
			if(count<nc) count = nc;
            foreach (var e in map[(int)now.Item1]) {
                que.In(new VT<long, int>(e,nc+1));
            }
        }
		return count;
    }







#region MyIO

public struct VT<T,U>
{
	public T Item1; public U Item2;
    public VT(T a,U b) { Item1 = a; Item2 = b; }
}
public struct VT<T,U,V>
{
	public T Item1; public U Item2; public V Item3;
    public VT(T a,U b,V c) { Item1 = a; Item2 = b; Item3 = c; }
}
public struct VT<T,U,V,W>
{
	public T Item1; public U Item2; public V Item3; public W Item4;
    public VT(T a,U b,V c,W d) { Item1 = a; Item2 = b; Item3 = c; Item4 = d; }
}

public static class MyIO
{
	class Prt : System.IO.StreamWriter
	{
		public override IFormatProvider FormatProvider { get { return System.Globalization.CultureInfo.InvariantCulture; } }
		public Prt(System.IO.Stream s) : base(s,new UTF8Encoding(false,true)) {}
		public Prt(System.IO.Stream s,Encoding e) : base(s,e) {}
	}
	
	static char[] sp = new char[] {' '};
	[MethodImpl(256)]
	static bool eq<T,U>() => typeof(T).Equals(typeof(U));
	[MethodImpl(256)]
	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,System.Globalization.CultureInfo.InvariantCulture))
	  : eq<T,char>()   ? ct<T,char>(s[0])
					   : ct<T,string>(s);
	public static string[] RM<T>(out T a) {
		var z = Console.ReadLine().Split(sp,StringSplitOptions.RemoveEmptyEntries); 
		a = cv<T>(z[0]);
		return z;
	}
	public static string[] RM<T,U>(out T a,out U b) {
		var z = RM<T>(out a); 
        b = cv<U>(z[1]);
		return z;
	}
	public static string[] RM<T,U,V>(out T a,out U b,out V c) {
		var z = RM<T,U>(out a,out b); 
        c = cv<V>(z[2]);
		return z;
	}
	public static string[] RM<T,U,V,W>(out T a,out U b,out V c,out W d) {
		var z = RM<T,U,V>(out a,out b,out c); 
        d = cv<W>(z[3]);
		return z;
	}
	public static string[] RM<T,U,V,W,X>(out T a,out U b,out V c,out W d,out X e) {
		var z = RM<T,U,V,W>(out a,out b,out c,out d); 
        e = cv<X>(z[4]);
		return z;
	}
	public static string[] RM<T,U,V,W,X,Y>(out T a,out U b,out V c,out W d,out X e,out Y f) {
		var z = RM<T,U,V,W,X>(out a,out b,out c,out d,out e); 
        f = cv<Y>(z[5]);
		return z;
	}
	public static string[] RA<T>(out T[] a) {		
		var z = Console.ReadLine().Split(sp,StringSplitOptions.RemoveEmptyEntries);
		a = z.Select(cv<T>).ToArray();
		return z;
	}
	public static string[][] RL<T>(long n,out T[] a) {
		a = new T[n];
		var y = new string[n][];
		for(int i=0;i<n;i++) y[i] = RM(out a[i]);
		return y;
	}
	public static string[][] RL<T,U>(long n,out VT<T,U>[] a) {
		a = new VT<T,U>[n];
		var y = new string[n][];
		for(int i=0;i<n;i++) {
			T o; U p;
			y[i] = RM(out o,out p);
			a[i] = new VT<T,U>(o,p);
		}
		return y;
	}
	public static string[][] RL<T,U,V>(long n,out VT<T,U,V>[] a) {
		a = new VT<T,U,V>[n];
		var y = new string[n][];
		for(int i=0;i<n;i++) {
			T o; U p; V q;
			y[i] = RM(out o,out p,out q);
			a[i] = new VT<T,U,V>(o,p,q);
		}
		return y;
	}
	public static string[][] RL<T,U,V,W>(long n,out VT<T,U,V,W>[] a) {
		a = new VT<T,U,V,W>[n];
		var y = new string[n][];
		for(int i=0;i<n;i++) {
			T o; U p; V q; W r;
			y[i] = RM(out o,out p,out q,out r);
			a[i] = new VT<T,U,V,W>(o,p,q,r);
		}
		return y;
	}
	public static string[][] RX<T>(long n,out T[][] a) {
		a = new T[n][];
		var y = new string[n][];
		for(int i=0;i<n;i++) y[i] = RA(out a[i]);
		return y;
	}

	static void wwp(Action act){
		var s = new Prt(Console.OpenStandardOutput()){AutoFlush = false};
		Console.SetOut(s);
		act();
		Console.Out.Flush();
		s.AutoFlush = true;
		Console.SetOut(s);
	}
	[MethodImpl(256)]
	static string wfm(Type t) =>t.Equals(typeof(double)) ? "{0:F10}" : "{0}";
	public static void WM(params object[] a) {
		wwp(()=>{
			for(int i=0;i<a.Length;i++) {
				var f = wfm(a[i].GetType()) + ((i<a.Length-1) ? " " : Environment.NewLine);
				Console.Write(f,a[i]);
			}
		});
	}
	public static void WA<T>(IList<T> a) {
		wwp(()=>{
			var f = wfm(typeof(T));
			var g = f + " ";
			var h = f + Environment.NewLine;
			for(int i=0;i<a.Count;i++) Console.Write(((i<a.Count-1) ? g : h),a[i]);
		});
	}
	public static void WL<T>(IList<T> a) {
		wwp(()=>{
			var f = wfm(typeof(T)) + Environment.NewLine;
			foreach(T x in a) Console.Write(f,x);
		});
	}


}

#endregion


}