杭电2094

#include <iostream>
#include <cstdio>
#include <cstring>
#include <map>
using namespace std;
int visit[5000];
int main()
{
	int n,i;
	string a,b;
	map<string,int>q;
	int cnt;
	while(~scanf("%d",&n)&&n)
	{
		cnt=1;
		q.clear();
		memset(visit,0,sizeof(visit));
		for(i=1;i<=n;i++)
		{
			cin>>a>>b;
			if(q[a]==0)q[a]=cnt++;
			if(q[b]==0)q[b]=cnt++;
			visit[q[b]]++;
		}
		int sum=0;
		for(i=1;i<cnt;i++)
		{
			if(visit[i]==0)sum++;
		}
		if(sum==1)cout<<"Yes"<<endl;
		else cout<<"No"<<endl;
	}
return 0;
}