// 10763 - Foreign Exchange
#include <iostream>
#define MAXN 500000
using namespace std;
int a[MAXN+1];

int main(void)
{
	int i, n, x, y;
	while(cin >> n && n != 0)
	{
		for(i = 1; i <= MAXN; i++)
			a[i] = i;

		for(i = 1; i <= n; i++)
		{
			cin >> x >> y;
			swap(a[x], a[y]);
		}

		for(i = 1; i <= MAXN; i++)
			if(a[i] != i)
			{
				cout << "NO" << endl;
				break;
			}

		if(i == MAXN+1)
			cout << "YES" << endl;
	}
	return 0;
}