UVa 10763 Foreign Exchange

#include<iostream>
#include<cstdio>
#include<string>
#include<map>
#include<sstream>
#include<algorithm>
#include<vector>
#include<queue>
#include<set>
#include<cstring>
#include<stack>
#include<iomanip>
#include<math.h>
using namespace std;

int main()
{
	int n;
	while(scanf("%d",&n)!=EOF&&n!=0)
	{
		int i,j;
		pair<int,int> temp;
		map<pair<int,int>,int> pair2int;
		vector<pair<int,int> > v;
		for(i=0;i<n;i++)
		{
			int a,b;
			scanf("%d %d",&a,&b);
			temp.first=a;
			temp.second=b;
			if(pair2int.count(temp)==0)
			{
				pair2int[temp]=1;
				v.push_back(temp);
			}
			else
				pair2int[temp]++;
		}
		bool istr=true;
		for(i=0;i<v.size();i++)
		{
			pair<int,int> temp2;
			temp2.first=v[i].second;
			temp2.second=v[i].first;
			if(pair2int[temp2]!=pair2int[v[i]])
			{
				istr=false;
				break;
			}
		}
		if(istr)
		{
			printf("YES\n");
		}
		else
		{
			printf("NO\n");
		}
	}
	return 0;
}