output_description: Print the number of operations that will be performed.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      H W
      A_{11}A_{12}...A_{1W}
      :
      A_{H1}A_{H2}...A_{HW}
attempt: H, W = list(map(int, input().split()))
  A = [input() for _ in range(H)]
  b = "#"
  w = "."
  
  wc = 0
  L = []
  wb = [[0] * W for _ in range(H)]
  for i in range(H):
      for j in range(W):
          if A[i][j] == w:
              wc += 1
          else:
              L.append([i, j])
              wb[i][j] = 1
  
  Ans = 0
  while wc > 0:
      L2 = []
      Ans += 1
      for ki, kj in L:
          if ki - 1 >= 0:
              if wb[ki - 1][kj] == 0:
                  wb[ki - 1][kj] = 1
                  wc -= 1
                  L2.append([ki - 1, kj])
          if ki + 1 < H:
              if wb[ki + 1][kj] == 0:
                  wb[ki + 1][kj] = 1
                  wc -= 1
                  L2.append([ki + 1, kj])
          if kj - 1 >= 0:
              if wb[ki][kj - 1] == 0:
                  wb[ki][kj - 1] = 1
                  wc -= 1
                  L2.append([ki, kj - 1])
          if kj + 1 < W:
              if wb[ki][kj + 1] == 0:
                  wb[ki][kj + 1] = 1
                  wc -= 1
                  L2.append([ki, kj + 1])
      L = L2
  print(Ans)
  
problem_description: Statement
  
  You are given a grid of squares with H horizontal rows and W vertical columns,
  where each square is painted white or black. HW characters from A_{11} to
  A_{HW} represent the colors of the squares. A_{ij} is `#` if the square at the
  i-th row from the top and the j-th column from the left is black, and A_{ij}
  is `.` if that square is white.
  
  We will repeatedly perform the following operation until all the squares are
  black:
  
    * Every white square that shares a side with a black square, becomes black.
  
  Find the number of operations that will be performed. The initial grid has at
  least one black square.
samples: [{"input": "3 3\n    ...\n    .#.\n    ...", "output": "2\n    \n\nAfter one operation, all but the corners of the grid will be black. After one\nmore operation, all the squares will be black.\n\n* * *"}, {"input": "6 6\n    ..#..#\n    ......\n    #..#..\n    ......\n    .#....\n    ....#.", "output": "3"}]