#pragma warning(disable : 4786)
#include <iostream>
#include <string>
#include <set>
using namespace std;
bool run()
{
int n;
cin>>n;
if (n==0) return false;
string s,t;
set <string> all,lose;
for(int i=0;i<n;i++)
{
cin>>s>>t;
all.insert(s);
all.insert(t);
lose.insert(t);
}
if (all.size()-lose.size()==1)
cout<<"Yes/n";
else
cout<<"No/n";
return true;
}