#include <bits/stdc++.h>
using namespace std;
int main(){
    int n;
    while(cin>>n&&n){
        set <string> player, loser;
        while(n--){
            string win,lose;
            cin>>win>>lose;
            player.insert(win);
            player.insert(lose);
            loser.insert(lose);
        }
        cout<< (player.size()==loser.size() + 1 ? "Yes":"No" )<<endl;
    }


}