1 #include <cstdio>
 2 #include <algorithm>
 3 #include <set>
 4 using namespace std;
 5 const int maxn=100000+10;
 6 int source[maxn];
 7 set<int>vis;
 8 int node[maxn];
 9 int finds(int t){
10     if(!source[t])
11         return t;
12     source[t]=finds(source[t]);
13     return source[t];
14 }
15 int main()
16 {
17     int a,b;
18     int yes;
19     while(scanf("%d%d",&a,&b)==2 && a>=0 && b>=0){
20         if(!a || !b){
21             printf("Yes\n");
22             continue;
23         }
24         vis.insert(a);
25         vis.insert(b);
26         for(int i=1;i<maxn;i++){
27             source[i]=0;
28             node[i]=1;
29         }
30         yes=1;
31         source[a]=b;
32         node[b]+=node[a];
33         int maxx=node[b];
34 
35         while(scanf("%d%d",&a,&b)==2 && a){
36             vis.insert(a);
37             vis.insert(b);
38             if(!yes)
39                 continue;
40             int s1=finds(a);
41             int s2=finds(b);
42             if(s1==s2){
43                 yes=0;
44                 continue;
45             }
46             source[s1]=s2;
47             node[s2]+=node[s1];
48             maxx=max(node[s2],maxx);
49         }
50         if(yes && maxx==vis.size())     printf("Yes\n");
51         else    printf("No\n");
52         vis.clear();
53     }
54     return 0;
55 }