#include <bits/stdc++.h>
#pragma warning(disable : 4996)
using namespace std;
int main() {
  char a[105][105];
  int n;
  while (cin >> n) {
    int sum = 0;
    for (int i = 0; i < n; i++)
      for (int j = 0; j < n; j++) {
        cin >> a[i][j];
        if (a[i][j] == '.') sum++;
      }
    if (sum % 5)
      cout << "NO" << endl;
    else {
      for (int i = 0; i < n; i++)
        for (int j = 0; j < n; j++)
          if (i > 0 && i < n - 1 && j > 0 && j < n - 1 && a[i][j] == '.') {
            if (a[i - 1][j] == '.' && a[i + 1][j] == '.' &&
                a[i][j + 1] == '.' && a[i][j - 1] == '.') {
              sum -= 5;
              a[i][j] = a[i - 1][j] = a[i + 1][j] = a[i][j - 1] = a[i][j + 1] =
                  '#';
            }
          }
      if (sum == 0)
        cout << "YES" << endl;
      else
        cout << "NO" << endl;
    }
  }
  return 0;
}
