#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;   }   