#include<bits/stdc++.h>
using namespace std;
int h,w;
int dx[]={0,0,1,-1};
int dy[]={1,-1,0,0};
char s[1005][1005];
struct node
{
    int x,y,res;
    node(int x=0,int y=0,int res=0):x(x),y(y),res(res){}
};
queue<node>q;
int main()
{
    scanf("%d%d",&h,&w);
    for(int i=1;i<=h;i++)
        scanf("%s",s[i]+1);
    for(int i=1;i<=h;i++)
        for(int j=1;j<=w;j++)
    {
        if(s[i][j]=='#')
            q.push(node(i,j,0));
    }
    int ans=0;
    while(!q.empty())
    {
        node now=q.front();q.pop();
        ans=max(ans,now.res);
        for(int i=0;i<4;i++)
        {
            int xx=dx[i]+now.x;
            int yy=dy[i]+now.y;
            if(xx<1||xx>h||yy<1||yy>w||s[xx][yy]=='#') continue;
            s[xx][yy]='#';
            q.push(node(xx,yy,now.res+1));
        }
    }
    cout<<ans<<endl;
}
