#include <bits/stdc++.h>

using namespace std;

int main()
{
    set<string>A,B;
    string s1,s2;
    int n;
    while(cin>>n&&n){
        for(int i =0 ;i < n; i++){
            cin>>s1>>s2;
            A.insert(s1);
            A.insert(s2);
            B.insert(s2);
        }
        if(A.size()-B.size()==1)
            cout<<"Yes"<<endl;
        else
            cout<<"No"<<endl; 
        A.clear();
        B.clear();
    }
    return 0;
}