#include <iostream>

using namespace std;

int par[101010];
int sum[101010];

int find(int x) {
	if (par[x] == x) return x;
	return par[x] = find(par[x]);
}

int in() {
	int x, y;
	cin >> x >> y;
	return x * 123 + y;
}

int main() {
	for (int i = 0; i < 101010; i++) {
		par[i] = i;
	}
	int n;
	cin >> n;
	for (int i = 0; i < n; i++) {
		int x = find(in());
		int y = find(in());
		if (x != y) {
			sum[x] += sum[y];
			par[y] = x;
		} else {
			sum[x]++;
		}
		if (sum[x] >= 2) {
			cout << "NO" << endl;
			return 0;
		}
	}
	cout << "YES" << endl;
}