#include <bits/stdc++.h>
void fun(std::string b[], int n) {
  int flag = 0;
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
      if (i < n - 2 && j < n - 1) {
        if (b[i][j] == '.') {
          if (b[i + 1][j] == '.' and b[i + 1][j - 1] == '.' and
              b[i + 1][j + 1] == '.' and b[i + 2][j] == '.') {
            b[i][j] = '#';
            b[i + 1][j] = '#';
            b[i + 1][j - 1] = '#';
            b[i + 1][j + 1] = '#';
            b[i + 2][j] = '#';
          } else {
            std::cout << "NO";
            return;
          }
        }
      } else if ((i >= n - 2 or j >= n - 1) and b[i][j] == '.') {
        std::cout << "NO";
        return;
      }
    }
  }
  std::cout << "YES";
  return;
}
int main() {
  int n;
  std::cin >> n;
  std::string board[n];
  for (int i = 0; i < n; i++) {
    std::cin >> board[i];
  }
  fun(board, n);
}
