RT，WA掉惹

#include<cstdio>
#include<map>
using namespace std;
map<long long,int>go;
map<long long,int>back;
int n,a[500005],b[500005];
bool check()
{
	for(int i=1;i<=n;++i)
		if(go[a[i]]!=back[a[i]])
			return false;
	return true;
}
int main()
{
	while(scanf("%d",&n)&&n!=0)
	{
		for(int i=1;i<=n;++i)
			scanf("%d%d",&a[i],&b[i]),go[a[i]]++,back[b[i]]++;
		if(check())
			printf("YES\n");
		else printf("NO\n");
	}
	return 0;
}