hdu 2094

#include <iostream>
#include <cstdio>
#include <cstring>
#include <string>
#include <cstdlib>
#include <cmath>
#include <vector>
#include <list>
#include <deque>
#include <queue>
#include <iterator>
#include <stack>
#include <map>
#include <set>
#include <algorithm>
#include <cctype>
using namespace std;

#define si1(a) scanf("%d",&a)
#define si2(a,b) scanf("%d%d",&a,&b)
#define sd1(a) scanf("%lf",&a)
#define sd2(a,b) scanf("%lf%lf",&a,&b)
#define ss1(s)  scanf("%s",s)
#define pi1(a)    printf("%d\n",a)
#define pi2(a,b)  printf("%d %d\n",a,b)
#define mset(a,b)   memset(a,b,sizeof(a))
#define forb(i,a,b)   for(int i=a;i<b;i++)
#define ford(i,a,b)   for(int i=a;i<=b;i++)

typedef long long LL;
const int N=1100001;
const int M=6666666;
const int INF=0x3f3f3f3f;
const double PI=acos(-1.0);
const double eps=1e-7;

int gcd(int a,int b)
{
    return b==0?a:gcd(b,a%b);
}

char s1[1005];
char s2[1005];
struct node
{
    char name[1005];
}a[2006];
map<string , int > mp;
map<string ,int >los;

int main()
{
    int num;
    while(~si1(num)&&num!=0)
    {
        int k=0;int sum=0;
        mset(a,0);
        mp.clear();
        los.clear();
        for(int i=0;i<num;i++)
        {
            scanf("%s%s",s1,s2);
            if(mp[s1]==0)
            {
                sum++;
                mp[s1]++;
                strcat(a[k++].name,s1);
            }
            if(mp[s2]==0)
            {
                sum++;
                mp[s2]++;
                strcat(a[k++].name,s2);
            }
            los[s2]=1;
        }
        int n=0;
        for(int i=0;i<k;i++)
        {
            if(los[a[i].name]==0)
                n++;
        }
       // printf("%d\n",n);
        if(n==1)
            printf("Yes\n");
        else
            printf("No\n");
    }
}