HDU 1269

#include<stdio.h>
#include<string.h>
#include<stack>
#include<string>
#include<math.h>
#include<queue>
#include<set>
#include<algorithm>
#include<iostream>
#include<vector>
#include<map>
using namespace std;
#define LL long long
#define inf 1<<30
#define N 10010
vector<int>edge[N];
int low[N],dfn[N],belong[N];
stack<int>s;
bool vis[N];
int Index,cnt;
void tarjan(int u)
{
    int i,v;
    low[u]=dfn[u]=++Index;
    s.push(u);
    vis[u]=1;
    for(i=0;i<edge[u].size();i++)
    {
        v=edge[u][i];
        if(dfn[v]==0)
        {
            tarjan(v);
            low[u]=min(low[u],low[v]);
        }
        else if(vis[v]) low[u]=min(low[u],dfn[v]);
    }
    if(low[u]==dfn[u])
    {
        cnt++;
        while(1)
        {
            v=s.top();s.pop();
            belong[v]=cnt;
            vis[v]=0;
            if(v==u) break;
        }
    }
}
int main()
{
    int i,j,n,m,u,v;
    while(scanf("%d%d",&n,&m),n+m)
    {
        for(i=1;i<=n;i++) edge[i].clear();
        for(i=0;i<m;i++)
        {
            scanf("%d%d",&u,&v);
            edge[u].push_back(v);
        }
        memset(dfn,0,sizeof(dfn));
        memset(vis,0,sizeof(vis));
        Index=cnt=0;
        for(i=1;i<=n;i++)
        {
            if(!dfn[i])
                tarjan(i);
        }
        if(cnt==1) printf("Yes\n");
        else printf("No\n");
    }
    return 0;
}