#include<bits/stdc++.h>
#define pii pair<int,int>
#define fi first
#define sc second
using namespace std;
const int N=1050;
const int dx[]={0,0,-1,1};
const int dy[]={-1,1,0,0};

int n,m;
char mp[N][N];
int dis[N][N],ans=0;

int main()
{
	scanf("%d%d",&n,&m);
	for(int i=1;i<=n;i++)
		scanf("%s",mp[i]+1);
	queue<pii>q;
	for(int i=1;i<=n;i++)
		for(int j=1;j<=m;j++)
			if(mp[i][j]=='#')dis[i][j]=1,q.push(pii(i,j));
	int x,y,nx,ny;
	while(!q.empty())
	{
		x=q.front().fi,y=q.front().sc,q.pop();
		ans=max(ans,dis[x][y]);
		for(int i=0;i<4;i++)
		{
			nx=x+dx[i],ny=y+dy[i];
			if(nx>=1&&nx<=n&&ny>=1&&ny<=m&&!dis[nx][ny])
				dis[nx][ny]=dis[x][y]+1,q.push(pii(nx,ny));
		}
	}
	printf("%d\n",ans-1);
}