要求严格a-b，b-a，所以分别对a，b排个序，然后比较是不是严格相同
/*
author:jxy
lang:C/C++
university:China,Xidian University
**If you need to reprint,please indicate the source**
*/
#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#define INF 1E9
using namespace std;
struct edge
{
int u,v;
friend bool operator ==(edge a,edge b)
{
return a.u==b.v&&a.v==b.u;
}
};
edge e[500005];
int a[500005],b[500005];
bool cmpa(int A,int B)
{
if(e[A].u==e[B].u)return e[A].v>e[B].v;
return e[A].u>e[B].u;
}
bool cmpb(int A,int B)
{
if(e[A].v==e[B].v)return e[A].u>e[B].u;
return e[A].v>e[B].v;
}
int main()
{
int n;
while(~scanf("%d",&n),n)
{
int i;
for(i=0;i<n;i++)
{
scanf("%d%d",&e[i].u,&e[i].v);
a[i]=b[i]=i;
}
sort(a,a+n,cmpa);
sort(b,b+n,cmpb);
for(i=0;i<n&&e[a[i]]==e[b[i]];i++);
if(i!=n)puts("NO");
else puts("YES");
}
}