#include <bits/stdc++.h>   using namespace std;   int dx[] = {0, 0, -1, 1};   int dy[] = {1, -1, 0, 0};   int main() {     int n;     char mp[100][100];     cin >> n;     for (int i = 0; i < n; ++i) cin >> mp[i];     auto isin = [&](int x) { return x >= 0 && x < n; };     auto isok = [&](int x, int y) {       for (int i = 0; i < 4; ++i) {         if (isin(x + dx[i]) && isin(y + dy[i])) {           if (mp[x + dx[i]][y + dy[i]] !=  . ) return 0;         } else           return 0;       }       mp[x][y] =  # ;       for (int i = 0; i < 4; ++i) {         mp[x + dx[i]][y + dy[i]] =  # ;       }       return 1;     };     for (int i = 0; i < n; ++i) {       for (int j = 0; j < n; ++j) {         if (mp[i][j] ==  . ) {           isok(i, j);         }       }     }     for (int i = 0; i < n; ++i) {       for (int j = 0; j < n; ++j) {         if (mp[i][j] ==  . ) {           cout <<  NO  << endl;           return 0;         }       }     }     cout <<  YES  << endl;     return 0;   }   