1 #include <stdio.h>
2 #include <iostream>
3 #include <algorithm>
4 #include <map>
5
6 using namespace std;
7
8 int main ()
9 {
10 int N;
11 while(cin >> N && N != 0)
12 {
13 map<int,int> m;
14 while(N--)
15 {
16 int a,b;
17 cin >> a >> b;
18 if(m.count(a))
19 {
20 m[a] --;
21 }
22 else
23 {
24 m[a] = -1;
25 }
26 if(m.count(b))
27 {
28 m[b] ++;
29 }
30 else
31 {
32 m[b] = 1;
33 }
34 }
35 int flag = 1;
36 for(auto Ptr = m.begin();Ptr != m.end();Ptr ++)
37 {
38 if(Ptr->second!=0)
39 {
40 flag = 0;
41 break;
42 }
43 }
44 if(flag)
45 cout << "YES" << endl;
46 else
47 cout << "NO" << endl;
48 }
49 return 0;
50 }
