1 #include <cstdio>
 2 #include <algorithm>
 3 #include <queue>
 4 using namespace std;
 5 const int maxn = 100050;
 6 
 7 int f[maxn];
 8 bool vis[maxn];
 9 typedef pair<int, int> P;
10 vector<P> v;
11 
12 int sf(int x){ return x == f[x] ? x : f[x] = sf(f[x]); }
13 int main()
14 {
15     int a, b, n = 0;
16     while (scanf("%d%d", &a, &b) == 2 && a != -1 && b != -1)
17     {
18         if (a&&b)
19         {
20             v.push_back(P(a, b));
21             n = max(a, n);
22             n = max(b, n);
23         }
24         else
25         {
26             bool flag = true;
27             int cnt = 0;//连通
28             if (n == 0) { printf("Yes\n"); continue; }
29             memset(vis, 0, sizeof(vis));
30             for (int i = 1; i <= n; i++)    f[i] = i;
31             for (vector<P>::iterator it = v.begin(); it != v.end(); it++)
32             {
33                 if (!vis[it->first]){ cnt++; vis[it->first] = true; }
34                 if (!vis[it->second]){ cnt++; vis[it->second] = true; }
35                 int q = sf(it->first); int w = sf(it->second);
36                 if (q != w){ cnt--;  f[q] = w; }
37                 else{ flag = false;  break; }
38             }
39             if (flag && cnt==1) printf("Yes\n");
40             else  printf("No\n");
41 
42             //清空和重置
43             v.clear();
44             n = 0;
45         }
46     }
47 }