Foreign Exchange

#include <iostream>
#include <vector>
#include <cstring>
#include <cmath>

using namespace std;

void solve();

int main()
{

    solve();
    return 0;
}

void solve()
{
    long long n, data_A, data_B;

    while(cin >> n)
    {
        if(n == 0)
        {
            break;
        }
        vector <long long> a;
        vector <long long> b;

        for(int i = 0; i < n; i++)
        {
            cin >> data_A;
            a.push_back(data_A);

            cin >> data_B;
            b.push_back(data_B);
        }

        bool check;
        for(int i = 0; i < n; i++)
        {
            if(a[i] != -1)
            {
                check = false;
                for(int j = 0; j < n; j++)
                {
                    if(a[i] == b[j] && a[j] == b[i])
                    {
                        a[i] = -1;
                        a[j] = -1;
                        b[i] = -1;
                        b[j] = -1;
                        check = true;
                        break;
                    }
                }
                if(check == false)
                {
                    break;
                }
            }
        }

        if(check == true)
        {
            cout << "YES" << endl;
        }
        else
        {
            cout << "NO" << endl;
        }
    }
}