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