Contest Duration: - (local time) (100 minutes) Back to Home
D - Maze Master /

Time Limit: 2 sec / Memory Limit: 1024 MB

### 制約

• 1 \leq H,W \leq 20
• S_{ij}.#
• S.2 つ以上含む
• 任意の道のマスから任意の道のマスまで 0 回以上の移動で到達できる

### 入力

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


### 入力例 1

3 3
...
...
...


### 出力例 1

4


### 入力例 2

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


### 出力例 2

10


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 ..

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.