#include <bits/stdc++.h>
using namespace std;
string A[51];
long long int n;
void tc(long long int x, long long int y) {
  if (x < 0 || y < 0 || x > n - 1 || y > n - 1 || A[x][y] == '#') {
    cout << "NO";
    exit(0);
  }
  A[x][y] = '#';
}
int main() {
  cin >> n;
  for (long long int i = 0; i < n; i++) cin >> A[i];
  for (long long int i = 0; i < n; i++) {
    for (long long int j = 0; j < n; j++) {
      if (A[i][j] == '.') {
        tc(i, j);
        tc(i + 1, j);
        tc(i + 1, j - 1);
        tc(i + 1, j + 1);
        tc(i + 2, j);
      }
    }
  }
  cout << "YES";
}
