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 = map(int, input().split())
  A = [["#"] + [s for s in input()] + ["#"] for k in range(H)]
  A = [["#"] * (W + 2)] + A + [["#"] * (W + 2)]
  
  
  que = []
  for k in range(1, H + 1):
      for l in range(1, W + 1):
          if A[k][l] == "#":
              que.append([k, l])
  if len(que) == H * W:
      print(0)
      exit(0)
  ans = 0
  while True:
      t = []
      while len(que) > 0:
          s = que.pop()
          k, l = s[0], s[1]
          if A[k][l - 1] == ".":
              A[k][l - 1] = "#"
              t.append([k, l - 1])
          if A[k][l + 1] == ".":
              A[k][l + 1] = "#"
              t.append([k, l + 1])
          if A[k - 1][l] == ".":
              A[k - 1][l] = "#"
              t.append([k - 1, l])
          if A[k + 1][l] == ".":
              A[k + 1][l] = "#"
              t.append([k + 1, l])
      if len(t) > 0:
          ans += 1
          que = t
      else:
          break
  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"}]