#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int n;
  cin >> n;
  string s[n];
  for (int i = 0; i < n; i++) cin >> s[i];
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
      if (s[i][j] == '.') {
        if (j == 0 || i >= n - 2 || j == n - 1 || s[i + 1][j] == '#' ||
            s[i + 1][j + 1] == '#' || s[i + 1][j - 1] == '#' ||
            s[i + 2][j] == '#') {
          cout << "NO"
               << "\n";
          exit(0);
        } else {
          s[i + 1][j] = '#';
          s[i + 1][j + 1] = '#';
          s[i + 1][j - 1] = '#';
          s[i + 2][j] = '#';
          s[i][j] = '#';
        }
      }
    }
  }
  cout << "YES"
       << "\n";
  return 0;
}
