#include <iostream>
#include <algorithm>
#include <queue>
#include <string>
#include <vector>
using namespace std;

int main()
{
    int n, m; cin>>n>>m;
    m+=2;
    string a=string(m, '.');
    for(int i=0; i<n; ++i) {
        string t; cin>>t; a+="."+t+".";
    }
    a+=string(m, '.');

    queue<int> q;
    vector<int> v(a.size(), 1<<30);
    vector<int> dir={-m-1, -m, -m+1, -1, 1, m-1, m, m+1};
    for(int i=0; i<a.size(); ++i) if (a[i]=='#') {
        bool f=false;
        for(int d: dir)
            if (a[i+d]=='.') f=true;
        if (f) {
            q.push(i);
            v[i]=1;
        }
    }
    
    while (q.size()) {
        int p=q.front(); q.pop();
        for(int d: dir) {
            int np=p+d;
            if (a[np]=='.' or v[np]<=v[p]+1) continue;
            q.push(np);
            v[np]=v[p]+1;
        }
    }
    int res=0;
    for(int i=0; i<a.size(); ++i) if (a[i]=='#' and res<v[i]) res=v[i];
    cout<<res<<endl;
}