#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  char d[111][111];
  cin >> n;
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= n; j++) {
      cin >> d[i][j];
    }
  }
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= n; j++) {
      if (d[i][j] == '.') {
        if (d[i][j + 1] == '.' && d[i - 1][j] == '.' && d[i + 1][j] == '.' &&
            d[i][j - 1] == '.') {
          d[i][j] = '#';
          d[i][j + 1] = '#';
          d[i - 1][j] = '#';
          d[i + 1][j] = '#';
          d[i][j - 1] = '#';
        }
      }
    }
  }
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= n; j++) {
      if (d[i][j] == '.') {
        cout << "NO";
        return 0;
      }
    }
  }
  cout << "YES";
  return 0;
}
