#include <bits/stdc++.h>
using namespace std;
int main(){
  int H, W;
  cin >> H >> W;
  vector<string> S(H);
  for (int i = 0; i < H; i++){
    cin >> S[i];
  }
  int tv = 1, fv = min(H, W) + 1;
  while (fv - tv > 1){
    int mid = (tv + fv) / 2;
    vector<vector<int>> A(H, vector<int>(W));
    for (int i = 0; i < H; i++){
      for (int j = 0; j < W; j++){
        if (S[i][j] == '.'){
          A[i][j] = 0;
        }
        if (S[i][j] == '#'){
          A[i][j] = 1;
        }
      }
    }
    vector<vector<int>> AS(H + 1, vector<int>(W + 1, 0));
    for (int i = 0; i < H; i++){
      for (int j = 0; j < W; j++){
        AS[i + 1][j + 1] += A[i][j];
      }
    }
    for (int i = 1; i <= H; i++){
      for (int j = 1; j <= W; j++){
        AS[i][j] += AS[i][j - 1];
      }
    }
    for (int i = 1; i <= W; i++){
      for (int j = 1; j <= H; j++){
        AS[j][i] += AS[j - 1][i];
      }
    }
    vector<vector<long long>> BS(H + 1, vector<long long>(W + 1, 0));
    for (int i = 0; i <= H - mid; i++){
      for (int j = 0; j <= W - mid; j++){
        int cnt = AS[i + mid][j + mid] - AS[i + mid][j] - AS[i][j + mid] + AS[i][j];
        if (cnt == mid * mid){
          BS[i][j]++;
          BS[i][j + mid]--;
          BS[i + mid][j]--;
          BS[i + mid][j + mid]++;
        }
      }
    }
    for (int i = 0; i <= H; i++){
      for (int j = 1; j <= W; j++){
        BS[i][j] += BS[i][j - 1];
      }
    }
    for (int i = 0; i <= W; i++){
      for (int j = 1; j <= H; j++){
        BS[j][i] += BS[j - 1][i];
      }
    }
    bool ok = true;
    for (int i = 0; i < H; i++){
      for (int j = 0; j < W; j++){
        if (A[i][j] == 1 && BS[i][j] == 0){
          ok = false;
        }
      }
    }
    if (ok){
      tv = mid;
    } else {
      fv = mid;
    }
  }
  cout << tv << endl;
}