#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<cmath>
#include<cstdlib>
#include<algorithm>
#include<queue>
#include<vector>
#include<stack>
using namespace std;
#define maxn 10000+10
vector<int> mp[maxn];
stack<int> s;
int n,m,Dfs[maxn],low[maxn];
bool isstack[maxn];
int use[maxn];
int top,newflag;
void init()
{
    memset(isstack,0,sizeof(isstack));
    memset(use,0,sizeof(use));
    memset(Dfs,0,sizeof(Dfs));
    memset(low,0,sizeof(low));
    for(int i=0;i<maxn;i++)
        mp[i].clear();
    while(!s.empty())
        s.pop();
    newflag=0;
    top=0;
}
void tarjan(int u)
{
    Dfs[u]=low[u]=++top;
    isstack[u]=1;
    s.push(u);
    for(int x=0;x<mp[u].size();x++)
    {
        int v=mp[u][x];
        if(!Dfs[v])
        {
            tarjan(v);
            if(low[v]<low[u])
                low[u]=low[v];
        }
        else if(isstack[v]&&Dfs[v]<low[u])
        {
            low[u]=Dfs[v];
        }
    }
    if(Dfs[u]==low[u])
    {
        newflag++;
        int x;
        do
        {
            x=s.top();
            isstack[x]=0;
            use[x]=newflag;
            s.pop();
        }while(x!=u);
    }
    return ;
}
int main()
{
    int x,y;
    while(scanf("%d%d",&n,&m)!=EOF)
    {
        if(n==0&&m==0)
            break;
        init();
        for(int i=1;i<=m;i++)
        {
            scanf("%d%d",&x,&y);
            mp[x].push_back(y);
        }
        for(int i=1;i<=n;i++)
        {
            if(!Dfs[i])
                tarjan(i);
        }
        if(newflag==1)
        {
            printf("Yes\n");
        }
        else
            printf("No\n");
    }
    return 0;
}