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 Ruby (2.3.3) (judge verdict: Accepted)

H, W = gets.chomp.split().map(&:to_i)
MAZE = Array.new(H)
H.times do |i|
  MAZE[i] = gets.chomp.chars
end

def bfs(start)
  queue = []
  costMemo = H.times.map { Array.new(W, -1) }
  #最初のキューを入れる (h, w, goal, cost)
  queue.push([start[0], start[1], 0])
  
  while !queue.empty? do
    farCost = 0
    #キューを取りだす
    loc = queue.shift
    h = loc[0]; w = loc[1]; cost = loc[2]
    #コストの記録を行い、既に短い経路が見つかっている場合はnext
    if costMemo[h][w] == -1 || costMemo[h][w] > cost 
      costMemo[h][w] = cost
    else
      next
    end
    
    #進行方向のキューを追加（上下、左右
    queue.push([h + 1, w, cost + 1]) if h < H - 1 && MAZE[h + 1][w] == '.'
    queue.push([h - 1, w, cost + 1]) if h > 0 && MAZE[h - 1][w] == '.'
    queue.push([h, w + 1, cost + 1]) if w < W - 1 && MAZE[h][w + 1] == '.'
    queue.push([h, w - 1, cost + 1]) if w > 0 && MAZE[h][w - 1] == '.'
  end
  
  #もっとも高かったコストを返す
  return costMemo.flatten.max
end

ans = -1

H.times do |i|
  W.times do |j|
    next if MAZE[i][j] == '#'
    mazeCost = bfs([i, j])
    ans = mazeCost if ans == -1 || ans < mazeCost
  end
end

puts ans