#include<iostream>
#include<cstdio>
#include<set>
using namespace std;
set<string> win,los;
int n;
int main()
{
      set<string>::iterator it;
      string a,b;
      while(scanf("%d",&n)!=EOF)
      {
            if(n==0) break;
            win.clear(),los.clear();
            for(int i=0;i<n;i++)
            {
                  cin>>a>>b;
                  win.insert(a);
                  los.insert(b);
            }
            for(it=los.begin();it!=los.end();it++)
                  if(win.count(*it))
                        win.erase(*it);
            if(win.size()==1)
                  printf("Yes\n");
            else
                  printf("No\n");
      }
      return 0;
}