#include <bits/stdc++.h>
using namespace std;
int main() {
    int n ;
    while(1) {
        freopen("input.txt","r",stdin);
        scanf("%d",&n);
        if ( n == 0) break ;
        int a,b;
        vector < pair <int,int> > vec;
        for ( int i = 0 ; i < n ; i++) {
            scanf("%d %d",&a,&b);
            vec.push_back(make_pair(min(a,b),max(a,b)));
        }
        sort(vec.begin(),vec.end());
        int fact = 0 ;
        for ( int i = 1 ; i < vec.size() ; i = i+2 ) {
            if (vec[i].first != vec[i-1].first || vec[i].second != vec[i-1].second) {
                    fact = 1 ;
                     break ;
         }
       }
       for ( int i = 0 ; i < vec.size() ; i++) {
        cout << vec[i].first << " " << vec[i].second << endl;
       }
       if ( fact == 1 ) printf("NO\n");
       else printf("YES\n");
    }
    return 0 ;
}