1 #include<iostream>
 2 #include<algorithm>
 3 #define Max 100005
 4 using namespace std;
 5
 6 int f[Max];
 7
 8 int root(int i)
 9 {
10     if(f[i] == i)
11         return i;
12     else
13         return root(f[i]);
14 }
15 int huan;
16 void U(int a,int b)
17 {
18     int f1,f2;
19     f1 = root(a);
20     f2 = root(b);
21     if(f1 != f2)
22     {
23         f[f1] = f2;
24     }
25     else
26     {
27         huan  = 1;//子环
28     }
29 }
30 int main()
31 {
32     int n,m,i,j,k,t;
33     while(~scanf("%d %d",&n,&m))
34     {
35         huan = 0;
36         memset(f,0,sizeof(f));
37         if(m == -1 && n == -1)
38             break;
39         else if(m == 0 && n == 0)
40         {
41             printf("Yes\n");
42             continue;
43         }
44         else
45         {
46             f[n] = n;
47             f[m] = m;
48             U(n,m);
49         }
50         while(~scanf("%d %d",&n,&m))
51         {
52             if(n == 0 && m == 0)
53                 break;
54             else
55             {
56                 if(f[n] == 0)
57                 {
58                     f[n] = n;
59                 }
60                 if(f[m] == 0)
61                 {
62                     f[m] = m;
63                 }
64                 U(n,m);
65             }
66         }
67         if(huan)
68         {
69             printf("No\n");
70             continue;
71         }
72         k = 0;
73         for(i = 0;i < Max;i ++)
74         {
75             if(f[i] == i && f[i] != 0) //只有一个点的根节点是他本身
76                 k ++;
77         }
78         if(k == 1)
79             printf("Yes\n");
80         else
81             printf("No\n");
82     }
83     return 0;
84 }