Problem:
Score : 300 points

Problem 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.

Constraints

1 \leq H,W \leq 1000

A_{ij} is # or ..

The given grid has at least one black square.

Input

Input is given from Standard Input in the following format:

H W
A_{11}A_{12}...A_{1W}
:
A_{H1}A_{H2}...A_{HW}

Output

Print the number of operations that will be performed.

Sample Input 1

3 3
...
.#.
...

Sample Output 1

2

After one operation, all but the corners of the grid will be black. After one more operation, all the squares will be black.

Sample Input 2

6 6
..#..#
......
#..#..
......
.#....
....#.

Sample Output 2

3

Solution in Perl (v5.18.2) (judge verdict: Time Limit Exceeded)

chomp (my ($h, $w) = split / /, <STDIN>);
my (@cm, @que);
for my $i (1..$h) {
  chomp (my $s = <STDIN>);
  for my $j (1..$w) {
    if (substr($s, $j - 1, 1) eq '.') {
      $cm[$i][$j] = -1;
    } else {
      push @que, [$i, $j];
    }
  }
}
my $ans = 0;
while(@que) {
  my $z = shift @que;
  my ($y, $x) = ($$z[0], $$z[1]);
  $ans = $cm[$y][$x] + 1;
  if ($cm[$y + 1][$x] == -1) {
    $cm[$y + 1][$x] = $ans;
    push @que, [$y + 1, $x];
  }
  if ($cm[$y - 1][$x] == -1) {
    $cm[$y - 1][$x] = $ans;
    push @que, [$y - 1, $x];
  }
  if ($cm[$y][$x + 1] == -1) {
    $cm[$y][$x + 1] = $ans;
    push @que, [$y, $x + 1];
  }
  if ($cm[$y][$x - 1] == -1) {
    $cm[$y][$x - 1] = $ans;
    push @que, [$y, $x - 1];
  }
}
print $ans - 1, "\n";