/*
Author:Chieh
Because Of Coding
*/
#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <set>
#define LL long long
#define INF 1e9
#define EPS 1e-9
using namespace std;
const int maxn=123;
//bool isC[maxn][maxn];
vector<int> V[maxn];
int n,E;
void init(){
    for(int i=1;i<=100;i++)V[i].clear();
    int u,v;
  //  memset(isC,0,sizeof(isC));
    for(int i=1;i<=n;i++){
        scanf("%d%d",&u,&v);
        if(u==v)continue;
        V[u].push_back(v);
     //   isC[u][v]=1;
    }
    /*for(int k=1;k<=100;k++){
        for(int i=1;i<=100;i++){
            for(int j=1;j<=100;j++){
                if(isC[i][k]&&isC[k][j])isC[i][j]=1;
            }
        }
    }*/
    scanf("%d",&E);
}
bool flag;
bool vis[maxn];
void DFS(int now){
    if(flag)return;
    for(int i=0;i<V[now].size();i++){
        int v=V[now][i];
        if(vis[v]){
            flag=1;
            return;
        }
        vis[v]=1;
        DFS(v);
        vis[v]=0;
    }
}
void play(){
    flag=0;
    memset(vis,0,sizeof(vis));
    vis[E]=0;
    DFS(E);
    if(flag)printf("No\n");
    else printf("Yes\n");
}
int main()
{
    while(scanf("%d",&n)!=EOF){
        if(n<0)break;
        init();
        play();
    }
//    cout << "Hello world!" << endl;
    return 0;
}