UVA 10763

#include <iostream>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <map>

using namespace std;
map<pair<int, int >, int > list;
int main()
{
    int n, a, b;
    while(scanf("%d",&n) != EOF && n)
    {
        list.clear();
        int cnt = 0;
        for( int i = 0; i < n; i++)
        {
            scanf("%d %d",&a , &b);
            if(!list[make_pair(a, b)])
                list[make_pair(a, b)] = 1;
            else
                list[make_pair(a, b)]++;


        }

        map<pair<int, int >,int >  :: iterator it;
        bool flag = true;
        for( it = list.begin(); it != list.end(); it++)
        {
           int a = it->first.first;
           int b = it->first.second;
           if(it->second != list[make_pair(b,a)])
               {
                   flag = false;
                   break;
               }
        }
        //cout<<cnt<<"   "<<ans<<endl;
        if(flag)
            printf("YES\n");
        else
            printf("NO\n");
    }
    return 0;
}