1 #include <iostream>
 2 #include <cstdio>
 3 #include <queue>
 4 #include <cstring>
 5 using namespace std;
 6 
 7 int father[100009];
 8 
 9 void set()
10 {
11     for (int i=1; i<100009; i++)
12         father[i]=i;
13 }
14 
15 int find(int a)
16 {
17     while (father[a]!=a)
18         a=father[a];
19     return a;
20 }
21 
22 int  Union(int x,int y)
23 {
24     x=find(x);
25     y=find(y);
26     if (x!=y)
27     {
28         father[x]=y;
29         return 0;
30     }
31     else
32         return 1;
33 }
34 
35 int main ()
36 {
37     int a,b,k,used[100009];
38     while (~scanf("%d%d",&a,&b))
39     {
40         if (a==0&&b==0)
41         {
42             printf ("Yes\n");
43             continue;
44         }
45         memset(used,0,sizeof(used));
46         queue<int>q;
47         q.push(a);
48         q.push(b);
49         used[a]=1;
50         used[b]=1;
51 
52         int flag=0,cmp=0;
53         set();
54         if (a==-1&&b==-1)
55             break;
56         Union(a,b);
57         while (scanf("%d%d",&a,&b),a||b)
58         {
59             if (Union(a,b)==1)
60                 flag=1;
61             if(used[a]!=1)
62             {
63                 q.push(a);
64                 used[a]=1;
65             }
66             if (used[b]!=1)
67             {
68                 q.push(b);
69                 used[b]=1;
70             }
71         }
72         while (!q.empty())
73         {
74             int s=q.front();
75             q.pop();
76             if (s==father[s])
77                 cmp++;
78         }
79         if (flag==0&&cmp==1)
80             printf ("Yes\n");
81         else
82             printf ("No\n");
83     }
84     return 0;
85 }