//  Created by conan1024hao in 2020.
//  Copyright © 2020 conan1024hao. All rights reserved.
//  専用ライブラリです、自由にコピーして構いません。
//  感谢看我的代码！Wechat:conan1024hao Twitter/QQ:810396815
#pragma GCC optimize ("O3")
#include <iostream>
#include <iomanip>
#include <istream>
#include <ostream>
#include <sstream>
#include <iterator>
#include <vector>
#include <algorithm>
#include <queue>
#include <deque>
#include <list>
#include <stack>
#include <map>
#include <unordered_map>
#include <set>
#include <utility>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <string>
#include <ctime>
#include <cctype>
#include <cstdlib>
#define IINF 10e8
#define INF 10e17
#define MOD 1000000007
#define mod 1000000007
#define rep(i, a, n) for (ll i = a; i < (ll)(n); i++)
#define Endl endl
#define fi first
#define se second
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define mmax(x,y)(x>y?x:y)
#define mmin(x,y)(x<y?x:y)
#define chmax(x,y) x=mmax(x,y)
#define chmin(x,y) x=mmin(x,y)
#define all(x) (x).begin(),(x).end()
#define siz(x) (ll)(x).size()
#define PI acos(-1.0)
using namespace std;
//using Int=int_fast64_t;
typedef long long int ll;
typedef long long int LL;
typedef pair<ll,ll>Pll;
typedef pair<int,int>Pin;
template<class T> using V=vector<T>;
long long GCD(long long a, long long b) { return b ? GCD(b, a%b) : a; }
long long LCM(long long a, long long b)  {return a/GCD(a,b)*b;}
int dx[4]={-1,0,1,0};
int dy[4]={0,-1,0,1};
int ddx[8]={-1,0,1,0,1,1,-1,-1};
int ddy[8]={0,-1,0,1,1,-1,1,-1};
char dir[4]={'u','l','d','r'};
ll cmp1(pair<ll,string>a,pair<ll,string> b){
        if(a.fi!=b.fi)
        return a.fi<b.fi;
        else
        return a.se<b.se;
}

//---------------------------------------------------------------------------
bool color[1001][1001];
//---------------------------------------------------------------------------
int main(){//問題をちゃんと見ろ!!!!!!!!!!!!!!!!!      llか??????????统一类型        memset()!!!!!!!!!!!!       ペナを減らせ!!!!!!!!!!!!!
    cin.tie(0);
    ios::sync_with_stdio(false);
    //-------------------------------
    int h,w;cin>>h>>w;
    char hw[1001][1001];
    int star=0;
    for(int i=0;i<h;i++){
        for(int j=0;j<w;j++){
            cin>>hw[i][j];
            if(hw[i][j]=='#')star++;
        }
    }
    if(star==h*w){
        cout<<0<<endl;
        return 0;
    }
    int cnt=0;
    int ans=0;
    int total=0;
    queue<Pin>xy;
    for(int i=0;i<h;i++){
        for(int j=0;j<w;j++){
            if(hw[i][j]=='#'){
                cnt++;
                xy.push({i,j});
                color[i][j]=1;
            }
        }
    }
    total+=cnt;
    while(1){
        ans++;
        
        int ncnt=0;
        while(cnt--){
            Pin now=xy.front();xy.pop();
            for(int i=0;i<4;i++){
                int nx=now.fi+dx[i];
                int ny=now.se+dy[i];
                if(nx>=0&&nx<h&&ny>=0&&ny<w){
                    if(hw[nx][ny]=='.'){
                        color[nx][ny]=1;
                        hw[nx][ny]='#';
                        xy.push({nx,ny});
                        ncnt++;
                    }
                }
            }
        }
        cnt=ncnt;
        total+=cnt;
        if(total==h*w)break;
    }
    cout<<ans<<endl;
    //-------------------------------
    return 0;
}
//---------------------------------------------------------------------------








Measured memory_bytes: 7296