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 Ruby (2.3.3) (judge verdict: Runtime Error)

require 'matrix'

M = 999999
h, w = gets.chomp!.split(" ").map!(&:to_i)
a_hw = Array.new(h) {|index| gets.chomp.chars } 
max_hw = Array.new(h) {|index| Array.new(w, M) }
 
black = []
a_hw.each_with_index do |arr, h2|
    arr.each_with_index do |ch, w2|
        if ch == "#"
            black.unshift([h2, w2, 0])
            max_hw[h2][w2] = 0
        end
    end
end
 
#i = 0
 
while black.empty?.!
    h2, w2, count = black.shift
    count += 1

    if h2 != 0 
        if max_hw[h2-1][w2] == M
            max_hw[h2-1][w2] = count
            black.push([h2 - 1, w2, count])
        end
    end
    if h2 != h - 1
        if max_hw[h2 + 1][w2] == M
            max_hw[h2+1][w2] = count
            black.push([h2 + 1, w2, count])
        end
    end
    if w2 != 0 
        if max_hw[h2][w2 - 1] == M
            max_hw[h2][w2 - 1] = count
            black.push([h2, w2 - 1, count])
        end
    end
    if w2 != w - 1
        if max_hw[h2][w2 + 1] == M
            max_hw[h2][w2 + 1] = count
            black.push([h2, w2 + 1, count])
        end
    end
    i += 1
end
 
#puts i
#puts max_hw.map {|x| x.join(",") }
puts max_hw.map{|x| x.max }.max