#include <bits/stdc++.h>
#define FOR(i,a,b) for (int i=(a);i<(b);i++)
#define FORR(i,a,b) for (int i=(a);i>=(b);i--)
#define pb push_back
#define pcnt __builtin_popcount
#define show(x) cout<<#x<<" = "<<x<<endl;
#define maxs(x,y) x = max(x,y)
#define mins(x,y) x = min(x,y)
#define fi first
#define se second
#define rng(a) a.begin(),a.end()
#define each(it,c) for(__typeof((c).begin()) it=(c).begin();it!=(c).end();it++)
#define sz(x) (int)(x).size()
#define mp make_pair

using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<pii> vpii;
typedef set<int> si;
typedef pair<ll,ll> pll;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<pll> vpll;
typedef set<ll> sl;
template<typename T>string join(vector<T>&v)
{stringstream s;FOR(i,0,sz(v))s<<' '<<v[i];return s.str().substr(1);}
ll gcd(ll a,ll b){if(a>b)swap(a,b);for(;a>0;b%=a,swap(a,b));return b;}
int modpow(ll a,ll n,int m){if(a==0)return a;ll p=1;for(;n>0;n/=2,a=a*a%m)if(n&1)p=p*a%m;return(int)p;}
void dout(double d){printf("%.12f\n",d);}

const int iinf = 1e9;
const ll linf = 1e18;
const int mod = 1e9+7;
const double pi = acos(-1);
const double eps = 1e-10;

int W, H;
string m[20];
set<pii> yy;
queue<pii> q, qq;
vpii V;
void sett(){
  FOR(i, 0, H)FOR(j, 0, W)if(m[i][j] == '.'){
    q.push(mp(i, j));
    yy.insert(mp(i, j));
    return;
  }
}
main(){
  cin.tie(0);
  ios::sync_with_stdio(false);
  cin >> W >> H;
  FOR(i, 0, H) cin >> m[i];
  V.pb(mp( 1, 0));
  V.pb(mp(-1, 0));
  V.pb(mp( 0, 1));
  V.pb(mp( 0,-1));
  sett();
  while(!q.empty()){
    pii t = q.front();
    q.pop();
    each(itr, V){
      int x = t.fi + itr->fi, y = t.se + itr->se;
      if(0 <= x && x < H && 0 <= y && y < W){
        pii tt = mp(x, y);
        if(yy.find(tt) == yy.end()){
          yy.insert(tt);
          if(m[x][y] == '.'){
            q.push(tt);
          }else{
            qq.push(tt);
          }
        }
      }
    }
  }
  int ans = 1;
  swap(q, qq);
  while(!q.empty()){
    while(!q.empty()){
      pii t = q.front();
      q.pop();
      each(itr, V){
        int x = t.fi + itr->fi, y = t.se + itr->se;
        if(0 <= x && x < H && 0 <= y && y < W){
          pii tt = mp(x, y);
          if(yy.find(tt) == yy.end()){
            yy.insert(tt);
            if(m[x][y] == '#'){
              qq.push(tt);
            }else{
              cout << ans << endl;
              return 0;
            }
          }
        }
      }
    }
    ans++;
    swap(q, qq);
  }
  return 0;
}