#include "bits/stdc++.h"
using namespace std;
#define ll long long
#define endl '\n'
#define rep(i, n) for(int i = 0; i < (int)(n); i++)
#define rep2(i,a,b) for(int i=int(a);i<int(b);i++)
#define pb emplace_back
#define all(x) (x).begin(),(x).end()
int mod=1e9+7;
int mod2=998244353;
const int INF=1e9;

signed main(){
	ll h,w;
	cin>>h>>w;
	vector<string> hw(h);
	rep(i,h)cin>>hw[i];

	queue<ll> qx;
	queue<ll> qy;
	rep(i,h){
		rep(j,w){
			if(hw[i][j]=='#'){
				qx.push(j);
				qy.push(i);
			}
		}
	}

	ll px[]={1,0,-1,0};
	ll py[]={0,-1,0,1};

	ll mp=qx.size();
	ll np=0;
	ll ans=0;
	bool f=false;
	while(!qx.empty()){
		ll x=qx.front();
		ll y=qy.front();
		qx.pop();
		qy.pop();
		rep(i,4){
			ll nx=x+px[i];
			ll ny=y+py[i];
			if(nx>=0&&ny>=0&&nx<w&&ny<h){
				if(hw[ny][nx]=='.'){
					f=true;
					qx.push(nx);
					qy.push(ny);
					hw[ny][nx]='#';
				}
			}
		}
		np++;
		if(np==mp){
			if(f)ans++;
			mp+=qx.size();
			f=false;
		}
	}
	cout<<ans<<endl;
}

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