#include "pch.h"

#include <string>

#include<iostream>

#include<map>

#include<memory.h>

#include<vector>

#include<algorithm>

#include<queue>

#include<vector>

namespace cc

{

	using std::cout;

	using std::endl;

	using std::cin;

	using std::map;

	using std::vector;

	using std::string;

	using std::sort;

	using std::priority_queue;

	using std::greater;

	using std::vector;

	using std::swap;

	constexpr int N = 500001;

	//constexpr int N = 30;

	//priority_queue<int,vector<int>, greater<int> >q;

	int mark[N];

	void init()

	{

		for (int i = 0;i < N;i++)

			mark[i] = i;

	}

	void solve()

	{

		int n, s, e;

		while (cin >> n && n)

		{

			init();

			for (int i = 0;i < n;i++)

			{

				cin >> s >> e;

				auto t = mark[e];

				mark[e] = mark[s];

				mark[s] = t;

			}

			int ok = 1;

			for (int i = 0;i < N;i++)

				if (mark[i] != i)

				{

					ok = 0;

					break;

				}

			if (ok)

				cout << "YES" << endl;

			else

				cout << "NO" << endl;

		}

	}

};

int main()

{

#ifndef ONLINE_JUDGE

	freopen("d://1.text", "r", stdin);

#endif // !ONLINE_JUDGE

	cc::solve();

	return 0;

}