#include <bits/stdc++.h>
using namespace std;
char a[100][100];
int main() {
  int n;
  memset(a, '#', sizeof(a));
  int flag = 0, sum = 0;
  int temp = 1;
  cin >> n;
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= n; j++) {
      cin >> a[i][j];
    }
  }
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= n; j++) {
      if (a[i][j] == '.') {
        if (a[i + 1][j] == '.' && a[i + 1][j + 1] == '.' &&
            a[i + 1][j - 1] == '.' && a[i + 2][j] == '.') {
          a[i + 1][j] = '#';
          a[i + 1][j + 1] = '#';
          a[i + 1][j - 1] = '#';
          a[i + 2][j] = '#';
          flag = 1;
        } else {
          flag = 0;
          printf("NO");
          goto here;
        }
      }
    }
  }
here:
  if (flag) printf("YES");
  return 0;
}
