#include <algorithm>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <functional>
#include <iostream>
#include <map>
#include <memory>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <vector>
#include <list>

using namespace std;

typedef long long ll;

#define sz size()
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define along long(c) (c).begin(), (c).end()
#define rep(i,a,b) for(ll i=(a);i<(b);++i)
#define clr(a, b) memset((a), (b) ,sizeof(a))
#define ctos(d) string(1,d)
#define print(x) cout<<#x<<" = "<<x<<endl;

#define MOD 1000000007

ll d[3003][3003];

int f(int y, int x) {
  if (y < 0 || x < 0)return 0;
  return d[y][x];
}

int main() {
  ll h, w;
  cin >> h >> w;
  vector<string> v;
  rep(i, 0, h) {
    string s;
    cin >> s;
    v.pb(s);
  }
  clr(d, 0);
  rep(y, 0, h) {
    rep(x, 0, w) {
      if (v[y][x] == '#') {
        d[y][x] = 1;
      }
    }
  }
  rep(y, 0, h) {
    rep(x, 1, w) {
      d[y][x] += d[y][x - 1];
    }
  }
  rep(y, 1, h) {
    rep(x, 0, w) {
      d[y][x] += d[y - 1][x];
    }
  }
  ll mx = 0;
  rep(y, 0, h) {
    rep(x, 0, w) {
      long long low = 0;
      long long high = 60;

      while (low < high) {
        long long mid = (high + low + 1) >> 1;

        ll count;
        count = f(y, x) - f(y - mid - 1, x) - f(y, x - mid - 1) + f(y - mid - 1, x - mid - 1);
        if (count == (mid + 1) * (mid + 1)) {
          low = mid;
        }
        else {
          high = mid - 1;
        }
      }
      mx = max(mx, (low + 2) / 2);
    }
  }
  cout << mx << endl;
  return 0;
}