#include <bits/stdc++.h>
using namespace std;
const int maxn = 100;
char ch[maxn][maxn];
int main() {
  int n;
  scanf("%d", &n);
  for (int i = 0; i <= n + 1; i++) {
    for (int j = 0; j <= n + 1; j++) {
      ch[i][j] = '#';
    }
  }
  for (int i = 1; i <= n; i++) {
    getchar();
    for (int j = 1; j <= n; j++) {
      scanf("%c", &ch[i][j]);
    }
  }
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= n; j++) {
      if (ch[i][j] == '#') {
        continue;
      }
      if (ch[i - 1][j] == '#') {
        continue;
      }
      if (ch[i][j - 1] == '#') {
        continue;
      }
      if (ch[i + 1][j] == '#') {
        continue;
      }
      if (ch[i][j + 1] == '#') {
        continue;
      }
      ch[i][j] = '#';
      ch[i][j + 1] = '#';
      ch[i][j - 1] = '#';
      ch[i + 1][j] = '#';
      ch[i - 1][j] = '#';
    }
  }
  bool ans = true;
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= n; j++) {
      if (ch[i][j] == '.') {
        ans = false;
      }
    }
  }
  if (ans) {
    printf("YES\n");
  } else {
    printf("NO\n");
  }
  return 0;
}
