#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  char arr[n + 2][n + 2];
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= n; j++) {
      cin >> arr[i][j];
    }
  }
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= n; j++) {
      if (arr[i][j] == '.') {
        if (arr[i + 1][j] != '.' || arr[i + 2][j] != '.' ||
            arr[i + 1][j - 1] != '.' || arr[i + 1][j + 1] != '.') {
          cout << "NO";
          return 0;
        } else {
          arr[i][j] = '#';
          arr[i + 1][j] = '#';
          arr[i + 2][j] = '#';
          arr[i + 1][j - 1] = '#';
          arr[i + 1][j + 1] = '#';
        }
      }
    }
  }
  cout << "YES";
  return 0;
}
