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