#include <stdio.h>
#include <iostream>
#include <string.h>
#include <algorithm>
#include <math.h>
using namespace std;
const int N = 100005;
struct Edge{
    int s,e;
}edge[N];
int father[N];
int Hash[N]; ///因为输入的树的编号不连续，离散化一下
int _find(int x){
    if(x==father[x]) return x;
    return _find(father[x]);
}
int Union(int a,int b){
    int x = _find(a);
    int y = _find(b);
    if(x==y) return 0;
    father[x]=y;
    return 1;
}
int main()
{
    int x,y;
    while(scanf("%d%d",&x,&y)!=EOF){
        if(x==-1&&y==-1) break;
        if(x==0&&y==0) { ///空树也是树
            printf("Yes\n"); continue;
        }
        for(int i=1;i<N;i++) father[i]=i;
        memset(Hash,-1,sizeof(Hash));
        int k=1,m=1;
        if(Hash[x]==-1) Hash[x]=k++;
        if(Hash[y]==-1) Hash[y]=k++;
        edge[m].s = Hash[x];
        edge[m++].e = Hash[y];
        while(scanf("%d%d",&x,&y)!=EOF){
            if(x==0&&y==0) break;
            if(Hash[x]==-1) Hash[x]=k++;
            if(Hash[y]==-1) Hash[y]=k++;
            edge[m].s = Hash[x];
            edge[m++].e = Hash[y];
        }
        bool flag = true;
        for(int i=1;i<m;i++){
            if(Union(edge[i].s,edge[i].e)==0){
                flag = false;
                break;
            }
        }
        int ans = 0;
        for(int i=1;i<k;i++){
            if(father[i]==i) ans++;
        }
        if(ans==1&&flag) printf("Yes\n");
        else printf("No\n");
    }
}