#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstring>
#include<algorithm>
#include<queue>
#include<map>
#include<set>
#include<vector>
#include<cstdlib>
#include<string>
#define eps 0.000000001
typedef long long ll;
typedef unsigned long long LL;
using namespace std;
const int N=100000+100;
int parent[N];
int vis[N];/*
int find(int x)
{
    if(x != parent[x])
        parent[x] = find(parent[x]);
    return parent[x];
}*/
int find(int x){
    int r=x;
    while(r!=parent[r])r=parent[r];
    int i=x;
    int j;
    while(parent[i]!=r){
        j=parent[i];
        parent[i]=r;
        i=j;
    }
    return r;
}
void Union(int x,int y){
    x=find(x);
    y=find(y);
    //if(x==y)return;
    if(x<y)parent[x]=y;
    else
        parent[y]=x;
}
int main(){
    int m,n;
    while(scanf("%d%d",&m,&n)!=EOF){
        int flag=1;
        memset(vis,0,sizeof(vis));
        for(int i=1;i<=N;i++)parent[i]=i;
        if(m==-1&&n==-1)break;
        while(1){
            if(m==0&&n==0)break;
            if(find(m)==find(n))flag=0;
            Union(m,n);
            vis[m]=1;
            vis[n]=1;
            cin>>m>>n;
        }
        if(flag==0){cout<<"No"<<endl;continue;}
        int ans=0;
        for(int i=1;i<=N;i++){
            if(vis[i]&&parent[i]==i)ans++;
        }
        if(ans>1)cout<<"No"<<endl;
        else{
            cout<<"Yes"<<endl;
        }
    }
}