#include<map>
#include<cmath>
#include<queue>
#include<stack>
#include<ctime>
#include<cctype>
#include<string>
#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef unsigned int uint;

const int maxn = 10000 + 5;
const int maxv = 100000 + 5;

int N,M,a,b;
vector<int>G[maxn]; //邻接表存图
stack<int>sta;      //存储已遍历的节点
int dfn[maxn];      //深度优先访问次序
int low[maxn];      //能追溯到的最早次序
bool InStack[maxn]; //是否在栈中
int index;          //索引号
int cnt;            //强连通分量个数

void init(){
    for(int i=1;i<=N;i++) G[i].clear();
    memset(dfn,0,sizeof(dfn));
    memset(low,0,sizeof(low));
    memset(InStack,false,sizeof(InStack));
    index=0; cnt=0;
    while(!sta.empty()) sta.pop();
}

void tarjan(int u){
    low[u]=dfn[u]=++index;
    sta.push(u);
    InStack[u]=true;
    for(int i=0;i<G[u].size();i++){
        int v=G[u][i];
        if(dfn[v]==0){
            tarjan(v);
            low[u]=min(low[u],low[v]);
        }
        if(InStack[v]){
            low[u]=min(low[u],dfn[v]);
        }
    }
    if(low[u]==dfn[u]){
        cnt++;
        while(!sta.empty()){
            int j=sta.top();
            sta.pop();
            InStack[j]=false;
            if(j==u) break;
        }
    }
}

void input(){
    for(int i=0;i<M;i++){
        scanf("%d%d",&a,&b);
        G[a].push_back(b);
    }
}

void solve(){
    for(int i=1;i<=N;i++){
        if(!dfn[i]) tarjan(i);
        if(cnt>1) break;
    }
    if(cnt>1) printf("No\n");
    else printf("Yes\n");
}

int main(){
    while(scanf("%d%d",&N,&M)!=EOF&&N+M){
        init();
        input();
        solve();
    }return 0;
}