#include <bits/stdc++.h>
using namespace std;
int dx[] = {0, 0, -1, 1};
int dy[] = {1, -1, 0, 0};
int main() {
  int n;
  char mp[100][100];
  cin >> n;
  for (int i = 0; i < n; ++i) cin >> mp[i];
  auto isin = [&](int x) { return x >= 0 && x < n; };
  auto isok = [&](int x, int y) {
    for (int i = 0; i < 4; ++i) {
      if (isin(x + dx[i]) && isin(y + dy[i])) {
        if (mp[x + dx[i]][y + dy[i]] != '.') return 0;
      } else
        return 0;
    }
    mp[x][y] = '#';
    for (int i = 0; i < 4; ++i) {
      mp[x + dx[i]][y + dy[i]] = '#';
    }
    return 1;
  };
  for (int i = 0; i < n; ++i) {
    for (int j = 0; j < n; ++j) {
      if (mp[i][j] == '.') {
        isok(i, j);
      }
    }
  }
  for (int i = 0; i < n; ++i) {
    for (int j = 0; j < n; ++j) {
      if (mp[i][j] == '.') {
        cout << "NO" << endl;
        return 0;
      }
    }
  }
  cout << "YES" << endl;
  return 0;
}
