#include<iostream>
#include<cstdio>
#include<string>
#include<algorithm>
#define MAXN 500005
using namespace std;
int arr[MAXN];
 
void swap(int a,int b){
    int t=arr[a]; arr[a] = arr[b]; arr[b] = t;
}

 
bool isOk(){
    for(int i=0; i<MAXN; ++i)
        if(arr[i]!=i) return false;
    return true;
}
 
int main(){
    int n, i, a, b;
    while(scanf("%d",&n), n){
        for(int i=0; i<MAXN; ++i)
            arr[i] = i;
        for(int i=0; i<n; ++i){
            scanf("%d %d", &a, &b);
            swap(a, b);
        }
        if(isOk()) printf("YES\n");
        else printf("NO\n");
    }
    return 0;
}