#include <bits/stdc++.h>
using namespace std;
void findOutcome() {
  int n;
  cin >> n;
  vector<string> arr;
  for (int i = 0; i < n; i++) {
    string s;
    cin >> s;
    arr.push_back(s);
  }
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
      if (arr[i][j] == '.') {
        arr[i][j] = '#';
        if (i + 1 < n && j < n && arr[i + 1][j] == '.')
          arr[i + 1][j] = '#';
        else {
          cout << "NO";
          return;
        }
        if (i + 1 < n && j - 1 >= 0 && arr[i + 1][j - 1] == '.')
          arr[i + 1][j - 1] = '#';
        else {
          cout << "NO";
          return;
        }
        if (i + 1 < n && j + 1 < n && arr[i + 1][j + 1] == '.')
          arr[i + 1][j + 1] = '#';
        else {
          cout << "NO";
          return;
        }
        if (i + 2 < n && j < n && arr[i + 2][j] == '.')
          arr[i + 2][j] = '#';
        else {
          cout << "NO";
          return;
        }
      }
    }
  }
  cout << "YES";
}
int main() {
  findOutcome();
  return 0;
}
