#include <bits/stdc++.h>
using namespace std;
char s[55][55];
int dx[] = {-1, 1, 0, 0}, dy[] = {0, 0, 1, -1}, n, vis[55][55];
bool check(int x, int y) {
  for (int i = 0; i < 4; i++) {
    int tx = x + dx[i], ty = y + dy[i];
    if (tx < 1 || tx > n || ty < 1 || ty > n || s[tx][ty] == '#' || vis[tx][ty])
      return false;
  }
  return true;
}
int main() {
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) {
    scanf("%s", s[i] + 1);
  }
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= n; j++) {
      if (vis[i][j] || s[i][j] == '#') continue;
      if (check(i, j)) {
        vis[i][j] = 1;
        for (int k = 0; k < 4; k++) vis[i + dx[k]][j + dy[k]] = 1;
      }
    }
  }
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= n; j++) {
      if (vis[i][j])
        continue;
      else if (s[i][j] == '#')
        continue;
      printf("NO\n");
      return 0;
    }
  }
  printf("YES\n");
  return 0;
}
