#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<cmath>
#include<algorithm>
#include<string>
#include<map>
#include<stack>
#include<set>
#include<iostream>
#include<vector>
#include<queue>
//ios_base::sync_with_stdio(false);
//#pragma comment(linker, "/STACK:1024000000,1024000000")

using namespace std;
#define rep(i, a, b) for (int i = (a); i < (b); ++i)
#define repf(i, a, b) for (int i = (a); i <= (b); ++i)
#define repd(i, a, b) for (int i = (a); i >= (b); --i)
#define clr(x) memset(x,0,sizeof(x))
#define sqr(x) ((x) * (x))
typedef long long LL;

const int INF = 1000000000;
const double eps = 1e-8;
const int maxn = 10000 + 10;
const int mod = 1000000000 + 7;
int vis[maxn];
int left_time[maxn];
int n,m;
vector<int> g[maxn];
int max_time;
void dfs(int n){
    if(vis[n]) return;
    vis[n] = 1;
    rep(i,0,g[n].size()){
        if(vis[g[n][i]] == 0){
            dfs(g[n][i]);
        }
    }
    left_time[++max_time] = n;
    return ;
}
void dfs_ans(int n){
    if(vis[n]) return ;
    vis[n] = 1;
    rep(i,0,g[n].size()){
        if(!vis[g[n][i]]){
            dfs_ans(g[n][i]);
        }
    }
    return ;
}
int ans(){
    int ret = 0;
    clr(vis);
    repf(i,1,max_time){
        if(vis[left_time[i]] == 0){
            ret++;
            if(ret > 1) return 1;
            dfs_ans(left_time[i]);
        }
    } 
    return 0;
}
int main() 
{
    //freopen("in.txt","r",stdin);
    while(scanf("%d%d",&n,&m) == 2){
        if(n == 0 && m == 0) break;
        max_time = 0;
        clr(vis);
        clr(left_time);
        repf(i,1,n) g[i].clear();
        rep(i,0,m){
            int a,b;
            scanf("%d%d",&a,&b);
            g[a].push_back(b);
        }
        dfs(1);
        int flag = 0;
        repf(i,1,n) if(vis[i] == 0) flag = 1;
        if(flag ||  ans()) printf("No\n");
        else printf("Yes\n");
    }
    return 0;
}