//set可以去重，并且自动排序

//数set元素个数要用 s.size() 的啊

#include<stdio.h>

#include<string.h>

#include<algorithm>

#include<iostream>

#include<string>

#include<math.h>

#include<set>

using namespace std;

//#define IN freopen("c:\\Users\\nit\\desktop\\input.txt", "r", stdin)

//#define OUT freopen("c:\\Users\\nit\\desktop\\output.txt", "w", stdout)

//int gcd(int a,int b){return b==0?a:gcd(b,a%b);}

int main()

{

    int n,i;

    string a,b;

    set<string>s1;

    set<string>s2;

    while(scanf("%d",&n),n)

    {

        s1.clear();

        s2.clear();

        for(i=;i<n;i++)

        {

            cin>>a>>b;

            s1.insert(a);//

            s1.insert(b);//把所有的都放在s1

            s2.insert(b);//把输过的都放在s2

        }

        if(s1.size()- s2.size() == )//比较s1和s2的不重复元素个数

            printf("Yes\n");

        else

            printf("No\n");

    }

    return ;

}