#include <bits/stdc++.h>
#define ll long long
#define INF 1000000005
#define MOD 1000000007
#define EPS 1e-10
#define rep(i,n) for(int i=0;i<(int)n;++i)
#define each(a,b) for(auto (a): (b))
#define all(v) (v).begin(),(v).end()
#define zip(v) sort(all(v)),v.erase(unique(all(v)),v.end())
#define fi first
#define se second
#define pb push_back
#define show(x) cout<<#x<<" = "<<(x)<<endl
#define spair(p) cout<<#p<<": "<<p.fi<<" "<<p.se<<endl
#define svec(v) cout<<#v<<":";rep(kbrni,v.size())cout<<" "<<v[kbrni];cout<<endl
#define sset(s) cout<<#s<<":";each(kbrni,s)cout<<" "<<kbrni;cout<<endl
#define smap(m) cout<<#m<<":";each(kbrni,m)cout<<" {"<<kbrni.first<<":"<<kbrni.second<<"}";cout<<endl

using namespace std;

typedef pair<int,int>P;

const int MAX_N = 3000;

const int dx[] = {1,0,0,-1};
const int dy[] = {0,1,-1,0};

string c[MAX_N];
int dir[MAX_N][MAX_N];

int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    int h,w;
    cin >> h >> w;
    rep(i,h){
        cin >> c[i];
    }
    rep(i,h){
        rep(j,w){
            dir[i][j] = INF;
        }
    }
    queue<P> que;
    rep(i,h){
        rep(j,w){
            if(c[i][j] == '.'){
                que.push(P(i,j));
                dir[i][j] = 0;
            }
        }
    }
    while(!que.empty()){
        P p = que.front();
        que.pop();
        rep(i,4){
            int u = p.fi+dx[i],v = p.se+dy[i];
            if(0 <= u && u < h && 0 <= v && v < w && dir[u][v] > dir[p.fi][p.se]+1){
                dir[u][v] = dir[p.fi][p.se]+1;
                que.push(P(u,v));
            }
        }
    }
    int ans = 0;
    rep(i,h){
        rep(j,w){
            ans = max(ans,dir[i][j]);
        }
    }
    cout << ans << endl;
    return 0;
}