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 Python (3.4.3) (judge verdict: Runtime Error)

H, W = map(int, input().split())
a = [[i for i in input().split()] for i in range(H)]
A = []
B = []

for i in range(H):
    word = a[i][0]
    for j in range(W):
        if word[j] == "#":
            A.append([i,j])
        else:
            B.append([i,j])

max = 0
for i in range(len(B)):
    x = B[i][0]
    y = B[i][1]
    min_temp = 0
    min = H * W
    for j in range(len(A)):
        min_temp = abs(x - A[k][0]) + abs(y - A[k][1])
        if min_temp <= min:
            min = min_temp
    if min >= max:
        max = min
print(max)