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

const short inf = 3e4;

int h, w;
int a[3002][3002];
string s[3002];
int main() {
  cin >> h >> w;
  s[0] = string(w + 2, '.');
  s[h + 1] = string(w + 2, '.');
  for (int i = 0; i < h; ++i) {
    string t;
    cin >> t;
    s[i + 1] = '.' + t + '.';
  }
  
  int mx = 0;
  for (int i = 1; i <= h; ++i) {
    for (int j = 1; j <= w; ++j) {
      if (s[i][j] == '#') {
        a[i][j] = min({ a[i - 1][j - 1], a[i - 1][j], a[i][j - 1] }) + 1;
        mx = max(mx, a[i][j]);
      }
    }
  }
  
  cout << (mx + 1) / 2 << endl;
}