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: Wrong Answer)

#include<stdio.h>
#include<string.h>
int main(){
  int i=0,n=0,m=0,h,w;
  scanf("%d %d",&h,&w);
  char a[h+1][w+1];
  int b[h][w];
  for(int s=0;s<h;s++)
      scanf("%s",a[s]);
  do{
    m=0;
  for(int s3=0;s3<h;s3++)
    for(int t3=0;t3<w;t3++)
        b[s3][t3]=0;
  for(int s2=0;s2<h;s2++)
    for(int t2=0;t2<w;t2++){
      if(a[s2][t2]=='#'&&b[s2][t2]==0){
        if(a[s2][t2+1]!='#')
          b[s2][t2+1]=1;
        a[s2][t2+1]='#';
        if(a[s2][t2-1]!='#')
          b[s2][t2-1]=1; 
        a[s2][t2-1]='#';
        if(a[s2+1][t2]!='#')
          b[s2+1][t2]=1; 
        a[s2+1][t2]='#';
        if(a[s2-1][t2]!='#')
          b[s2-1][t2]=1;
        a[s2-1][t2]='#';
      }
     }
   for(int s4=0;s4<h;s4++)
     for(int t4=0;t4<w;t4++){
      if(a[s4][t4]=='.')
      m++;
    }
   // printf("%d\n",m);
    n++;
  }while(m>0);
  printf("%d\n",n);
  return 0; 
}