output_description: Print the maximum possible score that Snuke can achieve, or print -1 if the
  game cannot be completed.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      H W
      s_{1, 1}s_{1, 2}s_{1, 3} ... s_{1, W}
      s_{2, 1}s_{2, 2}s_{2, 3} ... s_{2, W}
       :   :
      s_{H, 1}s_{H, 2}s_{H, 3} ... s_{H, W}
attempt: import sys
  
  h,w = [int(i) for i in input().split()]
  
  sys.setrecursionlimit(100000)
  
  s = [['#']*(w+2)] + [['#']+list(input())+['#'] for _ in range(h)] + [['#']*(w+2)]
  #print(s)
  
  cost = [[-1 if s[y][x] == '#' else float('inf') for x in range(w+2)] for y in range(h+2)]
  cost[1][1] = 0
  #print(cost)
  
  move_que = [[1,1]]
  
  def list_flatten(nested_list):
    flat_list = []
    fringe = [nested_list]
    
    while len(fringe) > 0:
      node = fringe.pop(0)
      if isinstance(node, list):
        fringe = node + fringe
      else:
        flat_list.append(node)
  
    return(flat_list)
        
  def move(place):
    y,x = place
    if cost[y+1][x] != -1　or cost[y+1][x] != float('inf'):
        cost[y+1][x] = cost[y][x]+1
        move_que.append([y+1,x])       
        
    if cost[y-1][x] != -1 or cost[y-1][x] != float('inf'):
        cost[y-1][x] = cost[y][x]+1
        move_que.append([y-1,x])
    
    if cost[y][x+1] != -1 or cost[y][x+1] != float('inf'):
        cost[y][x+1] = cost[y][x]+1
        move_que.append([y,x+1])
    
    if cost[y][x-1] != -1 or cost[y][x-1] != float('inf'):
        cost[y][x-1] = cost[y][x]+1
        move_que.append([y,x-1])
    
  while len(move_que) != 0:
    move(move_que[0])
    move_que = move_que[1:]
  
  #print(cost)
  
  if cost[h][w] == float('inf'):
    print(-1)
  else:
    while_num = list_flatten(s).count('.')
    print(while_num - (cost[h][w]+1))
  
problem_description: statement
  
  We have an H \times W grid whose squares are painted black or white. The
  square at the i-th row from the top and the j-th column from the left is
  denoted as (i, j).  
  Snuke would like to play the following game on this grid. At the beginning of
  the game, there is a character called Kenus at square (1, 1). The player
  repeatedly moves Kenus up, down, left or right by one square. The game is
  completed when Kenus reaches square (H, W) passing only white squares.  
  Before Snuke starts the game, he can change the color of some of the white
  squares to black. However, he cannot change the color of square (1, 1) and (H,
  W). Also, changes of color must all be carried out before the beginning of the
  game.  
  When the game is completed, Snuke's score will be the number of times he
  changed the color of a square before the beginning of the game. Find the
  maximum possible score that Snuke can achieve, or print -1 if the game cannot
  be completed, that is, Kenus can never reach square (H, W) regardless of how
  Snuke changes the color of the squares.
  
  The color of the squares are given to you as characters s_{i, j}. If square
  (i, j) is initially painted by white, s_{i, j} is `.`; if square (i, j) is
  initially painted by black, s_{i, j} is `#`.
samples: [{"input": "3 3\n    ..#\n    #..\n    ...", "output": "2\n    \n\nThe score 2 can be achieved by changing the color of squares as follows:\n\n![Explanation of Sample\n1](https://img.atcoder.jp/abc088/bc944898899615e35f898654b68cd517.png)\n\n* * *"}, {"input": "10 37\n    .....................................\n    ...#...####...####..###...###...###..\n    ..#.#..#...#.##....#...#.#...#.#...#.\n    ..#.#..#...#.#.....#...#.#...#.#...#.\n    .#...#.#..##.#.....#...#.#.###.#.###.\n    .#####.####..#.....#...#..##....##...\n    .#...#.#...#.#.....#...#.#...#.#...#.\n    .#...#.#...#.##....#...#.#...#.#...#.\n    .#...#.####...####..###...###...###..\n    .....................................", "output": "209"}]