#include<cstdio>
using namespace std;
const int maxn = 500000 + 10;
int A[maxn], B[maxn];
int main()
{
	int n;
	while(scanf("%d", &n) == 1)
	{
		if(n == 0) break;
		int ok = 0;
		for(int i = 1; i <= n; i++)
		{
			scanf("%d", &A[i]);
			scanf("%d", &B[i]);
		}
		for(int i = 1; i <= n; i++)
		{
			ok = 0;
			for(int j = 1; j <= n; j++)
				if(B[j] == A[i]) ok = 1;
			if(!ok) break;
		}
		printf("%s", ok ? "YES" : "NO");
	}
	return 0;
}

是因为没优化（虽然这题无关算法）还是因为啥（样例能过）