1 #include<bits/stdc++.h>
 2 using namespace std;
 3 int pre[100001],r[100001];
 4 bool vis[100001];
 5 bool flag=0;
 6 int findset(int x) {
 7     int r=x;
 8     while(pre[r]!=r) r=pre[r];
 9     int i=x,j;
10     while(i!=r) {
11         j=pre[i];
12         pre[i]=r;
13         i=j;
14     }
15     return r;
16 }
17 void join(int f1,int f2) {
18     if(r[f1]<=r[f2]) {
19         pre[f1]=f2;
20         if (r[f1]==r[f2]) r[f2]++;
21     }
22     else pre[f2]=f1;
23 }
24 int main() {
25     int a,b;
26     while(~scanf("%d%d",&a,&b)) {
27         for (int i=1;i<=100000;++i) pre[i]=i,r[i]=1;
28         memset(vis,0,sizeof(vis));
29         if (a==-1&&b==-1) break;
30         if (a==0&&b==0) {
31             printf("Yes\n");
32             continue;
33         }
34         int sign=0,minn=1000001,maxn=-1;
35         do {
36             minn=min(min(minn,a),b);
37             maxn=max(max(maxn,a),b);
38             vis[a]=vis[b]=1;
39             int f1=findset(a),f2=findset(b);
40             if(f1==f2) sign=-1;
41             else join(f1,f2);
42             scanf("%d%d",&a,&b);
43             if(a==0&&b==0) break;
44         }while(1);
45         if(sign==-1) {
46             printf("No\n");
47             continue;
48         }
49         bool x=0;
50         for (; minn<=maxn; minn++)
51             if(vis[minn] && pre[minn]==minn) {
52                 if(x==1) {x=0;break;}
53                 else x=1;
54             }
55         if(x) printf("Yes\n");
56         else printf("No\n");
57     }
58     return 0;
59 }