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 C (GCC 5.4.1) (judge verdict: Time Limit Exceeded)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
int check(char **s, int h, int w, int hei, int wid) {
    int end = (h > w) ? h : w;
    int a = 0, b = 0, c = 0;
    for (int i=1; i<=end; i++) {
        for (int j=-i; j<=i; j++) {
            if (((c = wid+j) > w-1) || (c < 0)) continue;
            if (!((a = hei+i-abs(j)) > h-1)) {
                if (s[a][c] == '#') return i;
            }
            if (!((b = hei-i+abs(j)) < 0)) {
                if (s[b][c] == '#') return i;
            }
        }
    }
    return -1;
}
 
int main() {
    int h = 0, w = 0;
    scanf("%d %d\n", &h, &w);
    char s[h][w+2];
    char *s_ch[h];
    for (int i=0; i<h; i++) {
        fgets(s[i], w+2, stdin);
        s_ch[i] = s[i];
    }
    int max = 0;
    for (int i=0; i<h; i++) {
        for (int j=0; j<w; j++) {
            if (s[i][j] == '.') {
                int buf = check(s_ch, h, w, i, j);
                if (buf > max) max = buf;
            }
        }
    }
    printf("%d\n", max);
    return 0;
}