#include <cstdio>
#include <cstring>
#include <string>
#include <cmath>
#include <cassert>
#include <iostream>
#include <algorithm>
#include <stack>
#include <queue>
#include <vector>
#include <set>
#include <map>
#include <bitset>
using namespace std;

#define repl(i,a,b) for(int i=(int)(a);i<(int)(b);i++)
#define rep(i,n) repl(i,0,n)
#define mp(a,b) make_pair(a,b)
#define pb(a) push_back(a)
#define all(x) (x).begin(),(x).end()
#define dbg(x) cout<<#x"="<<x<<endl

#define INF 1<<30

typedef pair<int, pair<int, int> > P;

int mat[3005][3005];

int main(){
    int h,w;
    cin>>h>>w;

    fill(mat[0], mat[3005], INF);

    queue<P> que;
    char c[3005];
    rep(i,h){
        scanf("%s", c);
        rep(j,w){
            if(c[j]=='.'){
                mat[i][j] = 0;
                que.push(mp(0,mp(i,j)));
            }
        }
    }

    // insert out of map
    rep(i,h+2){ que.push(mp(0,mp(i-1, -1))); que.push(mp(0,mp(i-1, w)));}
    rep(i,w  ){ que.push(mp(0,mp(-1, i))); que.push(mp(0,mp(h, i)));}

    while(!que.empty()){
        const int di[] = {-1,1,0}, dj[]={0,-1,1};
        P p = que.front(); que.pop();
        int nl = p.first+1;
        int i = p.second.first, j = p.second.second;
        rep(ri, 3) rep(rj,3){
            int ni = i+di[ri], nj = j+dj[rj];
            if(ni<0 || ni>=h || nj<0 || nj>=w) continue;

            if(mat[ni][nj] > nl){
                mat[ni][nj] = nl;
                que.push(mp(nl, mp(ni,nj)));
            }
        }
    }

    //rep(i,h){rep(j,w) printf("%2d", mat[i][j]); printf("\n");}

    int res=0;
    rep(i,h)rep(j,w) res = max(res, mat[i][j]);

    cout<<res<<endl;

    return 0;
}