#include <bits/stdc++.h>
using namespace std;
void ri(int& x);
const int MAXN = 52;
int n;
char mp[MAXN][MAXN];
bool check(int x, int y) {
  if (y - 1 < 0 || y + 1 == n || x + 2 == n) return false;
  return mp[x][y] == '.' && mp[x + 1][y] == '.' && mp[x + 1][y - 1] == '.' &&
         mp[x + 1][y + 1] == '.' && mp[x + 2][y] == '.';
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cin >> n;
  for (int i = 0; i < n; ++i) {
    cin >> mp[i];
  }
  bool ok = true;
  for (int i = 0; i < n; ++i)
    for (int j = 0; j < n; ++j)
      if (mp[i][j] == '.') {
        if (!check(i, j)) {
          ok = false;
          goto FL;
        } else {
          mp[i][j] = mp[i + 1][j] = mp[i + 1][j - 1] = mp[i + 1][j + 1] =
              mp[i + 2][j] = '#';
        }
      }
FL:
  cout << (ok ? "YES" : "NO") << endl;
  return 0;
}
void ri(int& x) {
  char c;
  bool minus = false;
  while ((c = getchar()) < '0' || '9' < c)
    if (c == '-') minus = true;
  x = c - '0';
  while ('0' <= (c = getchar()) && c <= '9') x = 10 * x - '0' + c;
  if (minus) x = -x;
}
