#include <bits/stdc++.h>
using namespace std;
int main() {
  cin.tie(0); cout.tie(0);
  ios::sync_with_stdio(false);
  int H, W;
  cin >> H >> W;
  vector<string> S(H);
  for(int i = 0; i < H; i++) {
    cin >> S[i];
  }
  vector<vector<int>> C(H + 1, vector<int>(W + 1));
  for(int i = 0; i < H; i++) {
    for(int j = 0; j < W; j++) {
      C[i + 1][j + 1] = C[i + 1][j] + C[i][j + 1] - C[i][j] + (S[i][j] == '#' ? 1 : 0);
    }
  }
  int ok = 1, ng = min(H, W) + 1;
  while(ng - ok > 1) {
    int m = (ok + ng) >> 1;
    vector<vector<int>> imos(H + 1, vector<int>(W + 1));
    for(int i = 0; i <= H - m; i++) {
      for(int j = 0; j <= W - m; j++) {
        if(C[i + m][j + m] - C[i][j + m] - C[i + m][j] + C[i][j] == m * m) {
          imos[i][j]++;
          imos[i][j + m]--;
          imos[i + m][j]--;
          imos[i + m][j + m]++;
        }
      }
    }
    for(int i = 0; i < H; i++) {
      for(int j = 1; j < W; j++) {
        imos[i][j] += imos[i][j - 1];
      }
    }
    for(int i = 1; i < H; i++) {
      for(int j = 0; j < W; j++) {
        imos[i][j] += imos[i - 1][j];
      }
    }
    bool fn = true;
    //cout << m << endl;
    for(int i = 0; i < H; i++) {
      for(int j = 0; j < W; j++) {
        //cout << imos[i][j];
        if(S[i][j] == '.' && imos[i][j] > 0) {
          fn = false;
        }
        if(S[i][j] == '#' && imos[i][j] <= 0) {
          fn = false;
        } 
      }
      //cout << endl;
    }

    if(fn) {
      ok = m;
    } else {
      ng = m;
    }
  }
  cout << ok << '\n';
  return 0;
}