#include<bits/stdc++.h>
#include<iostream>
#include<cmath>
#include<vector>
#include<string>
#include<queue>
#include<set>
#include<map>
#include<algorithm>
#include<functional>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef pair<ll, ll> pll;
typedef pair<ull, ull> pullull;
typedef pair<ll, int> plli;
typedef pair<int, pii> pipii;
typedef vector<vector<int> > mati;
typedef vector<vector<double> > matd;
typedef vector<ll> vll;
typedef vector<vector<ll>> vvll;
typedef vector<vector<vector<ll>>> vvvll;
typedef vector<bool> vb;
typedef vector<vector<bool>> vvb;
typedef vector<vector<vector<bool>>> vvvb;

#define FOR(i,x,y) for(ll i=(ll)x; i<(ll)y; ++i)
#define REP(i,y) FOR(i, 0, y)
#define RFOR(i,x,y) for(ll i=(ll)x; i>=(ll)y; --i)
#define RREP(i,x) RFOR(i, x, 0)

template <typename T>
void vec_print(vector<T> VEC){
  REP(i, VEC.size()){
    cout << VEC[i] << " ";
  }
  cout << "\n";
};

template <typename T>
void mat_print(vector<vector<T> > MAT){
  REP(i, MAT.size()){
    REP(j, MAT[i].size()){
      cout << MAT[i][j] << " ";
    }
    cout << "\n";
  }
};

template <typename CLASS1, typename CLASS2>
class HOGE{
  public:
    CLASS1 key;
    CLASS2 value;
    HOGE(void){
      return;
    };
    HOGE(CLASS1 key, CLASS2 value){
      this->key = key;
      this->value = value;
    };
    ~HOGE(void){
      return;
    };

    void print(void){
      cout << "key : " << key << ", value : " << value << "\n";
      return;
    };
    
    bool operator==(const HOGE &obj){
      return (this->value == obj.value);
    };
    bool operator<(const HOGE &obj){
      return (this->value < obj.value);
    };
    bool operator>(const HOGE &obj){
      return (this->value > obj.value);
    };
};

constexpr int INF = (1<<30);
constexpr ll INFLL = 1LL<<62;
constexpr long double EPS = 1e-12;
constexpr ll MOD = (ll)((1E+9)+7);

ll H, W;
vvll A;

int main(){
  cin.tie(0); // cut the cin and cout (default, std::flush is performed after std::cin)
  ios::sync_with_stdio(false); // cut the iostream and stdio (DON'T endl; BUT "\n";)

  cin >> H >> W;
  A.resize(H);
  queue<pair<ll, ll> > que;

  REP(i, H){
    A[i].resize(W, INFLL);
    REP(j, W){
      char tmp;
      cin >> tmp;
      if(tmp=='#'){
        pair<ll, ll> tmp(i, j);
        que.push(tmp);
        A[i][j] = 0;
      }
    }
  }

  while(!que.empty()){
    pair<ll, ll> pos = que.front(); que.pop();
    ll x = pos.first;
    ll y = pos.second;
    ll dist = A[x][y];
    if(x>0){
      if(A[x-1][y] > A[x][y]+1){
        pair<ll, ll> tmp(x-1, y);
        que.push(tmp);
        A[x-1][y] = A[x][y]+1;
      }
    }
    if(x<H-1){
      if(A[x+1][y] > A[x][y]+1){
        pair<ll, ll> tmp(x+1, y);
        que.push(tmp);
        A[x+1][y] = A[x][y]+1;
      }
    }
    if(y>0){
      if(A[x][y-1] > A[x][y]+1){
        pair<ll, ll> tmp(x, y-1);
        que.push(tmp);
        A[x][y-1] = A[x][y]+1;
      }
    }
    if(y<W-1){
      if(A[x][y+1] > A[x][y]+1){
        pair<ll, ll> tmp(x, y+1);
        que.push(tmp);
        A[x][y+1] = A[x][y]+1;
      }
    }
  }

  ll ans = 0;
  REP(i, H){
    REP(j, W){
      ans = max(ans, A[i][j]);
    }
  }

  cout << ans << "\n";

  return 0;
}

Measured memory_bytes: 24500