#include <bits/stdc++.h>
using namespace std;
const int N = 51, P = 1e9 + 7;
int a[N][N];
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  string s;
  for (int i = 0; i < n; i++) {
    cin >> s;
    for (int j = 0; j < n; j++) {
      if (s[j] == '#')
        a[i][j] = 1;
      else
        a[i][j] = 0;
    }
  }
  for (int i = 1; i < n - 1; i++) {
    for (int j = 1; j < n - 1; j++) {
      if (!a[i][j]) {
        if (!a[i - 1][j] and !a[i + 1][j] and !a[i][j - 1] and !a[i][j + 1]) {
          a[i - 1][j] = a[i + 1][j] = a[i][j] = a[i][j - 1] = a[i][j + 1] = 1;
        }
      }
    }
  }
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
      if (!a[i][j]) {
        cout << "NO\n";
        return 0;
      }
    }
  }
  cout << "YES\n";
  return 0;
}
