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 (Clang 3.8.0) (judge verdict: Wrong Answer)

// Twitter@KonoLv1 2020年春からエンジニア見習いデビュー！
 
//  標準入出力を扱うヘッダーファイルの読み込み
#include <stdio.h>
//  "atoi" "atof"などのキャスト、"rand" 疑似乱数を発生させるヘッダーファイルの読み込み
#include <stdlib.h>
//  "strcpy" "strcat"などの文字列操作や"strlen"文字列長さ取得などを行うヘッダーファイルの読み込み
#include <string.h>
//  "sqrtl" "fmax"などの数値操作を行うヘッダーファイルの読み込み
#include <math.h>

 
int main(void){
  //  char text[256];
//    int no[256];
  //  int yuukoudo[500][500];
    char s[256] = {};
    int ok[100001] = {};
    int x,y,z;
    int i,j;
    int pi;
    int n,m;
    int ng[100001] = {};
    int flg = 0;
    int ngcount = 0;
    int count = 0;
    int h,w;
    int world;
    int pin_x = 0;
    int pin_y = 0;
    int jama = 0;
    char date[2569][2569]={};
    scanf("%d %d",&h,&w);

    for (i = 0; i <= h; i++){
        for (j = 0; j <= w; j++){
            scanf("%c",&date[i][j]);
        }
    }
    
    for (i = 0; i <= h; i++){
        for (j = 0; j <= w; j++){
        }
    }
     
     
    for (i = 0; i <= h; i++){
        for (j = 0; j <= w; j++){
            if (date[i][j] == '#'){
                jama++;
            }
        }
    }
    if (jama == 0){
        printf("%d",h - 1 + w - 1);
    }else{
        printf("%d",h * w - 1 - jama);
    }
     
   
}