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 Java8 (OpenJDK 1.8.0) (judge verdict: Time Limit Exceeded)

import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayDeque;
import java.util.Arrays;
import java.util.NoSuchElementException;
import java.util.Queue;
import java.util.Scanner;
class FastScanner {
	private final InputStream in = System.in;
	private final byte[] buffer = new byte[1024];
	private int ptr = 0;
	private int buflen = 0;
	private boolean hasNextByte() {
		if (ptr < buflen) {
			return true;
		}else{
			ptr = 0;
			try {
				buflen = in.read(buffer);
			} catch (IOException e) {
				e.printStackTrace();
			}
			if (buflen <= 0) {
				return false;
			}
		}
		return true;
	}
	private int readByte() { if (hasNextByte()) return buffer[ptr++]; else return -1;}
	private static boolean isPrintableChar(int c) { return 33 <= c && c <= 126;}
	public boolean hasNext() { while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++; return hasNextByte();}
	public String next() {
		if (!hasNext()) throw new NoSuchElementException();
		StringBuilder sb = new StringBuilder();
		int b = readByte();
		while(isPrintableChar(b)) {
			sb.appendCodePoint(b);
			b = readByte();
		}
		return sb.toString();
	}
	public long nextLong() {
		if (!hasNext()) throw new NoSuchElementException();
		long n = 0;
		boolean minus = false;
		int b = readByte();
		if (b == '-') {
			minus = true;
			b = readByte();
		}
		if (b < '0' || '9' < b) {
			throw new NumberFormatException();
		}
		while(true){
			if ('0' <= b && b <= '9') {
				n *= 10;
				n += b - '0';
			}else if(b == -1 || !isPrintableChar(b)){
				return minus ? -n : n;
			}else{
				throw new NumberFormatException();
			}
			b = readByte();
		}
	}
	public int nextInt() {
		long nl = nextLong();
		if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException();
		return (int) nl;
	}
	public double nextDouble() { return Double.parseDouble(next());}
	public char nextchar() {
		try { return (char)System.in.read(); } catch(Exception e) {
			throw new RuntimeException(e);
		}
	}
}
public class Main {
	static FastScanner scan=new FastScanner();
	static Scanner scanner=new Scanner(System.in);
	static long mod=1000000007;
	static double eps=0.0000000001;
	static int big=Integer.MAX_VALUE;
	static long gcd (long a, long b) {return b>0?gcd(b,a%b):a;}
	static long lcm (long a, long b) {return a*b/gcd(a,b);}
	static int max(int a,int b) {return a>b?a:b;}
	static int min(int a,int b) {return a<b?a:b;}
	static long lmax(long a,long b) {return Math.max(a, b);}
	static long lmin(long a,long b) {return Math.min(a, b);}
	static long factorial(int i) {return i==1?1:i*factorial(i-1);}
	static int lower_bound(int a[],int key) {
		int right=a.length;
		int left=0;
		while(right-left>0) {
			int mid=(right+left)/2;
			if(a[mid]<key) {
				left=mid;
			}
			else {
				right=mid;
			}
		}
		return left;
	}
	static int upper_bound(int a[],int key) {
		int right=a.length;
		int left=0;
		while(right-left>0) {
			int mid=(right+left)/2;
			if(a[mid]<=key) {
				left=mid;
			}
			else {
				right=mid;
			}
		}
		return left;
	}
	static boolean isPrime (long n) {
		if (n==2) return true;
		if (n<2 || n%2==0) return false;
		double d = Math.sqrt(n);
		for (int i=3; i<=d; i+=2)if(n%i==0){return false;}
		return true;
	}
	static int upper_division(int a,int b) {
		if(a%b==0) {
			return a/b;
		}
		else {
			return a/b+1;
		}
	}
	static long lupper_division(long a,long b) {
		if(a%b==0) {
			return a/b;
		}
		else {
			return a/b+1;
		}
	}
	static int[] setArray(int a) {
		int b[]=new int[a];
		for(int i=0;i<a;i++) {
			b[i]=scan.nextInt();
		}
		return b;
	}
	static long[] lsetArray(int a) {
		long b[]=new long[a];
		for(int i=0;i<a;i++) {
			b[i]=scan.nextLong();
		}
		return b;
	}
	static String reverce(String str) {
		String strr="";
		for(int i=str.length()-1;i>=0;i--) {
			strr+=str.charAt(i);
		}
		return strr;
	}
	public static void printArray(int[] que) {
		for(int i=0;i<que.length-1;i++) {
			System.out.print(que[i]+" ");
		}
		System.out.println(que[que.length-1]);
	}
	public static int[][] doublesort(int[][]a) {
		Arrays.sort(a,(x,y)->Integer.compare(x[0],y[0]));
		return a;
	}
	static long modpow(long x,long n,long mo) {
		long sum=1;
		while(n>0) {
			if((n&1)==1) {
				sum=sum*x%mo;
			}
			x=x*x%mo;
			n>>=1;
		}
		return sum;
	}
	public static char[] revch(char ch[]) {
		char ret[]=new char[ch.length];
		for(int i=ch.length-1,j=0;i>=0;i--,j++) {
			ret[j]=ch[i];
		}
		return ret;
	}
	public static int[] revint(int ch[]) {
		int ret[]=new int[ch.length];
		for(int i=ch.length-1,j=0;i>=0;i--,j++) {
			ret[j]=ch[i];
		}
		return ret;
	}
	public static void warshall_floyd(int v[][],int n) {
		for(int k=0;k<n;k++)
			for(int i=0;i<n;i++)
				for(int j=0;j<n;j++)
					v[i][j]=min(v[i][j],v[i][k]+v[k][j]);
	}
	public static long modinv(long a,long m) {
		long b=m,u=1,v=0;
		while(b!=0) {
			long t=a/b;
			a-=t*b;
			long x=a;
			a=b;
			b=x;
			
			u-=t*v;
			x=u;
			u=v;
			v=x;
		}
		u%=m;
		if(u<0)u+=m;
		return u;
	}
	public static void main(String[] args) {
		int n=scan.nextInt();
		int m=scan.nextInt();
		char ch[][]=new char[n][m];
		for(int i=0;i<n;i++)ch[i]=scan.next().toCharArray();
		Queue<Integer> X=new ArrayDeque<>();
		Queue<Integer> Y=new ArrayDeque<>();
		Queue<Integer> Cost=new ArrayDeque<>();
		int ans=0;
		for(int i=0;i<n;i++) {
			for(int j=0;j<m;j++) {
				if(ch[i][j]=='#')continue;
				X=new ArrayDeque<>();
				Y=new ArrayDeque<>();
				Cost=new ArrayDeque<>();
				X.add(i);
				Y.add(j);
				Cost.add(0);
				char c[][]=new char[n][m];
				for(int z=0;z<n;z++)c[z]=ch[z].clone();
				int COST=0;
				int cc[][]=new int[n][m];
				for(;;) {
					if(X.size()==0) {
						break;
					}
					int x=X.poll();
					int y=Y.poll();
					int cost=Cost.poll();
					cc[x][y]=cost;
					c[x][y]='#';
					if(x!=0) {
						if(c[x-1][y]=='.') {
							X.add(x-1);
							Y.add(y);
							Cost.add(cost+1);
						}
					}
					if(x!=n-1) {
						if(c[x+1][y]=='.') {
							X.add(x+1);
							Y.add(y);
							Cost.add(cost+1);
						}
					}
					if(y!=0) {
						if(c[x][y-1]=='.') {
							X.add(x);
							Y.add(y-1);
							Cost.add(cost+1);
						}
					}
					if(y!=m-1) {
						if(c[x][y+1]=='.') {
							X.add(x);
							Y.add(y+1);
							Cost.add(cost+1);
						}
					}
				}
				for(int x=0;x<n;x++) {
					for(int y=0;y<m;y++) {
						COST=max(COST,cc[x][y]);
					}
				}
				ans=max(ans,COST);
			}
		}
		System.out.println(ans);
	}
}