6 8  5 3  5 2  6 4
5 6  0 0

8 1  7 3  6 2  8 9  7 5
7 4  7 8  7 6  0 0

3 8  6 8  6 4
5 3  5 6  5 2  0 0

-1 -1

Yes
Yes
No

#include<cstdio>
#include<iostream>
#include <cstring>
#include<set>
using namespace std;
#define maxn 1000001
int father[maxn];
void init()
{
    for(int i=0;i<maxn;i++)
        father[i]=i;
}
int Find(int x)
{
    return x==father[x]?x:father[x]=Find(father[x]);
}
void Union(int x,int y)
{
    int fx=Find(x);
    int fy=Find(y);
    if(fx!=fy)
        father[fy]=fx;
}
int main()
{
    //freopen("in.txt", "r", stdin);
    int a,b;
    set<int> s;
    set<int>::iterator ite;;
    while(scanf("%d%d",&a,&b)!=EOF&&(a!=-1&&b!=-1))
    {
        int c=1;
        init();
        s.clear();
        int flag=1;
        if(a==0&&b==0)
        {
            cout<<"Yes"<<endl;
            continue;
        }
        s.insert(a);
        s.insert(b);
        Union(a,b);
        while(scanf("%d%d",&a,&b)!=EOF)
        {
            if(a==0&&b==0)
            {
                for(ite=s.begin();ite!=s.end();ite++)
                    if(*ite==father[*ite])
                        c++;
                break;
            }
            else{
        if(Find(a)==Find(b))
            flag=0;
        else
            Union(a,b);
        }
        }
        if(!flag||c!=1)
                cout<<"No"<<endl;
        else
                cout<<"Yes"<<endl;
    }
    return 0;
}