#include <bits/stdc++.h>
using namespace std;
bool c[51][51];
long long int n;
bool check(long long int x, long long int y) {
  if (x >= 0 && x <= n - 3 && y >= 1 && y <= n - 2) {
    if (!c[x][y] && !c[x + 1][y] && !c[x + 1][y - 1] && !c[x + 1][y + 1] &&
        !c[x + 2][y]) {
      c[x][y] = c[x + 1][y] = c[x + 1][y - 1] = c[x + 1][y + 1] = c[x + 2][y] =
          1;
      return 1;
    }
  }
  return 0;
}
int main() {
  memset(c, 0, sizeof(c));
  cin >> n;
  for (int i = 0; i < n; i++) {
    string s;
    cin >> s;
    for (int j = 0; j < n; j++) {
      if (s[j] == '#') c[i][j] = 1;
    }
  }
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
      if (!c[i][j]) {
        if (!check(i, j)) return cout << "NO", 0;
      }
    }
  }
  cout << "YES";
}
