////  1450 产生冠军.cpp//  Jobdu////  Created by PengFei_Zheng on 21/04/2017.//  Copyright © 2017 PengFei_Zheng. All rights reserved.// #include <stdio.h>#include <iostream>#include <algorithm>#include <string.h>#include <cmath>#include <map>#define MAX_SIZE 2017 using namespace std; map<string,int> name2id;int inDegree[MAX_SIZE]; int n; int main(){    while(scanf("%d",&n)!=EOF && n!=0){        for(int i = 1 ; i <= 2*n ; i++){            inDegree[i]=0;        }        while(!name2id.empty()) name2id.clear();        string a, b;        int num = 0;        while(n--){            cin>>a>>b;            if(!name2id.count(a)){                num++;                name2id.insert(make_pair(a, num));            }            if(!name2id.count(b)){                num++;                name2id.insert(make_pair(b, num));            }            int id_b = name2id[b];            inDegree[id_b]++;        }        int ans = 0;        for(int i = 1 ; i <= num ; i++){            if(inDegree[i]==0)                ans++;        }        ans == 1 ? printf("Yes\n") : printf("No\n");    }}/**************************************************************    Problem: 1450    User: zpfbuaa    Language: C++    Result: Accepted    Time:40 ms    Memory:1536 kb****************************************************************/