#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int n;
  cin >> n;
  vector<string> a(n);
  for (int i = 0; i < n; ++i) cin >> a[i];
  bool ans = true;
  for (int i = 0; i < n; ++i) {
    for (int j = 0; j < n; ++j) {
      if (a[i][j] == '.') {
        a[i][j] = '#';
        if (i + 1 >= n || a[i + 1][j] != '.') {
          ans = false;
          break;
        }
        a[i + 1][j] = '#';
        if (i + 1 >= n || j - 1 < 0 || a[i + 1][j - 1] != '.') {
          ans = false;
          break;
        }
        a[i + 1][j - 1] = '#';
        if (i + 1 >= n || j + 1 >= n || a[i + 1][j + 1] != '.') {
          ans = false;
          break;
        }
        a[i + 1][j + 1] = '#';
        if (i + 2 >= n || a[i + 2][j] != '.') {
          ans = false;
          break;
        }
        a[i + 2][j] = '#';
      }
    }
    if (ans == false) break;
  }
  if (ans)
    cout << "YES";
  else
    cout << "NO";
  return 0;
}
