#include<bits/stdc++.h>
using namespace std;
#define int long long

#ifdef LOCAL_DEBUG
  #include "LOCAL_DEBUG.hpp"
#endif

char table[3003][3003];
int mp[3003][3003];
queue<pair<int,int>> q;
const int INF = 1 << 30;

signed main(){

  int h,w; cin >> h >> w;
  for(int i = 0; i <= h+1; i++){
    for(int j = 0; j <= w+1; j++){
      if(i == 0 || j == 0 || i == h+1 || j == w+1){
        q.push({i,j});
        continue;
      }
      char c; scanf("%c",&c);
      table[i][j] = c;
      if(table[i][j] == '.'){
        q.push({i,j});
      }
      else mp[i][j] = INF;
     }
  }

  while(!q.empty()){
    auto p = q.front(); q.pop();
    for(int dx : {-1,0,1}){
      for(int dy : {-1,0,1}){
        int ny = p.first + dy;
        int nx = p.second + dx;
        if(ny < 0 || h+1 < ny || nx < 0 || w+1 < nx) continue;
        if(mp[ny][nx] == INF && !(dx == 0 && dy == 0)) q.push({ny,nx});
        if((ny == 1 || ny == h || nx == 1 || nx == w) && table[ny][nx] == '#') mp[ny][nx] = 1;
        if(1 <= ny && ny <= h && 1 <= nx && nx <= w){
          mp[p.first][p.second] = min(mp[p.first][p.second],mp[ny][nx]+1);
        }
      }
    }
  }

  int ans = 0;
  for(int i = 0; i <= h; i++){
    for(int j = 0; j <= w; j++){
      ans = max(ans,mp[i][j]);
    }
  }
  cout << ans << endl;

  return 0;
}