#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=; 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=; i<MAXN; ++i)

            arr[i] = i;

        for(int i=; i<n; ++i){

            scanf("%d %d", &a, &b);

            swap(a, b);

        }

        if(isOk()) printf("YES\n");

        else printf("NO\n");

    }

    return ;

}