#include <bits/stdc++.h>
using namespace std;
int main() {
  long long m, n, t, b, c, d, i, j, k, x, y, z, l, q, r;
  long long cnt = 0, ans = 0;
  scanf("%lld", &n);
  char a[n][n];
  for (long long i = 0; i < n; i++) {
    for (long long j = 0; j < n; j++) {
      cin >> a[i][j];
    }
  }
  for (long long i = 0; i < n; i++) {
    for (long long j = 0; j < n; j++) {
      if (a[i][j] == '.') {
        a[i][j] = '#';
        if (i + 1 < n && j < n && a[i + 1][j] == '.') {
          a[i + 1][j] = '#';
        } else {
          cout << "No";
          return 0;
        }
        if (i + 1 < n && j + 1 < n && a[i + 1][j + 1] == '.') {
          a[i + 1][j + 1] = '#';
        } else {
          cout << "No";
          return 0;
        }
        if (i + 1 < n && j - 1 >= 0 && a[i + 1][j - 1] == '.') {
          a[i + 1][j - 1] = '#';
        } else {
          cout << "No";
          return 0;
        }
        if (i + 2 < n && j < n && a[i + 2][j] == '.') {
          a[i + 2][j] = '#';
        } else {
          cout << "No";
          return 0;
        }
      }
    }
  }
  cout << "YES" << endl;
  return 0;
}
