#include<iostream>
#include<map>
#include<vector>
#include<cmath>
#include<string>
#include<cstring>
#include<algorithm> 
#include<climits>
#include<stack>
#include<queue>
#include<set>
#include<cctype>
#include<bitset> 
#include <type_traits>
using namespace std;

# define REP(i,n) for (int i=0;i<(n);++i)
# define rep(i,a,b) for(int i=a;i<(b);++i)
# define p(s) std::cout << s ;
# define pl(s)  std::cout << s << endl;
# define printIf(j,s1,s2) cout << (j ? s1 : s2) << endl;
# define YES(j) cout << (j ? "YES" : "NO") << endl;
# define Yes(j) std::cout << (j ? "Yes" : "No") << endl;
# define yes(j) std::cout << (j ? "yes" : "no") << endl;
# define all(v) v.begin(),v.end()
# define showVector(v) REP(i,v.size()){p(v[i]);p(" ")} pl("")
template<class T> inline bool chmin(T &a, T b){ if(a > b) { a = b; return true;} return false;}
template<class T> inline bool chmax(T &a, T b){ if(a < b) { a = b; return true;} return false;}
typedef long long int ll;
typedef pair<ll,ll> P_ii;
typedef pair<double,double> P_dd;

template<class T>
vector<T> make_vec(size_t a){
    return vector<T>(a);
}

template<class T, class... Ts>
auto make_vec(size_t a, Ts... ts){
  return vector<decltype(make_vec<T>(ts...))>(a, make_vec<T>(ts...));
}

template<typename T,typename V>
typename enable_if<is_class<T>::value==0>::type
fill_v(T &t,const V &v){t=v;}

template<typename T,typename V>
typename enable_if<is_class<T>::value!=0>::type
fill_v(T &t,const V &v){
  for(auto &e:t) fill_v(e,v);
}

const int mod = 1000000007;
const int MOD = 1000000007;
const int inf=1e9+7;
const ll longinf=1LL<<60 ;

void addM(long long &a, long long b) {
    a += b;
    if (a >= MOD) a -= MOD;
}

void mulM(long long &a, long long b) {
    a = ((a%MOD)*(b%MOD))%MOD ;
}

// a^b mod M
long myPow(long a,long b,int M) {
    long ret = 1;
    long tmp = a;
    while(b>0) {
        if((b&1)==1) ret = (ret * tmp) % M;
        tmp = (tmp * tmp) % M;
        b = b>>1;
    }
    return ret;
}

 // nCk mod M
int nCk(int n,int k,int M) {
    long ret = 1;
    int mi = min(k, n-k);
    for(int i=1;i<=mi;i++) {
        ret = (ret * myPow(i,M-2,M)) % M;
    }
    for(int i=n-mi+1;i<=n;i++) {
        ret = (ret * i) % M;
    }
    return (int)ret;
}

int main(){

    int H,W;cin >> H >> W;

    auto graph = make_vec<char>(H, W);

    queue<P_ii> que;
    queue<P_ii> n_que;

    int w_sum = 0;
    REP(i,H) REP(j,W) {
        cin >> graph[i][j];
        if(graph[i][j]=='#') {
            que.push(make_pair(i,j));
        }
    }

    int ans = 0;
    int di[4] = {1,-1,0,0};
    int dj[4] = {0,0,1,-1};
    while(!que.empty()){
        
        while(!que.empty()){
            P_ii p = que.front();que.pop();            

            REP(i,4){
                int ni = p.first  + di[i];
                int nj = p.second + dj[i];
                if( 0 <= ni && 0 <= nj && ni < H && nj < W && graph[ni][nj]=='.'){
                    n_que.push(make_pair(ni,nj));
                    graph[ni][nj]='#';
                }
            }
        }
        while(!n_que.empty()) {
            que.push(n_que.front());
            n_que.pop();
        }
        if(que.size()==0) break;
        ans++;
    }
    pl(ans)
    return 0;
}

Measured memory_bytes: 17588