#include <map>
#include <set>
#include <list>
#include <queue>
#include <stack>
#include <vector>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>

using namespace std;

const int N = 1010;
int n;
char str1[110], str2[110];
int in[N];

int main()
{
	while (~scanf("%d", &n), n)
	{
		map<string, int>qu;
		memset (in, 0, sizeof(in));
		qu.clear();
		int cnt = 0;
		for (int i = 1; i <= n; ++i)
		{
			scanf("%s%s", str1, str2);
			if (qu[str1] == 0)
			{
				qu[str1] = ++cnt;
			}
			if (qu[str2] == 0)
			{
				qu[str2] = ++cnt;
			}
			in[qu[str2]]++;
		}
		int ans = 0;
		for (int i = 1; i <= cnt; ++i)
		{
			if (in[i] == 0)
			{
				ans++;
			}
		}
		if (ans >= 2 || ans == 0)
		{
			printf("No\n");
			continue;
		}
		printf("Yes\n");
	}
	return 0;
}