UVA-10763 Foreign Exchange

#include <stdio.h>
#include <iostream>
#include <algorithm>
#include <map>

using namespace std;

int main ()
{
    int N;
    while(cin >> N && N != 0)
    {
        map<int,int> m;
        while(N--)
        {
            int a,b;
            cin >> a >> b;
            if(m.count(a))
            {
                m[a] --;
            }
            else
            {
                m[a] = -1;
            }
            if(m.count(b))
            {
                m[b] ++;
            }
            else
            {
                m[b] = 1;
            }
        }
        int flag = 1;
        for(auto Ptr = m.begin();Ptr != m.end();Ptr ++)
        {
            if(Ptr->second!=0)
            {
                flag = 0;
                break;
            }
        }
        if(flag)
            cout << "YES" << endl;
        else
            cout << "NO" << endl;
    }
    return 0;
}