/*

     Name:hdoj-3342-Legal or Not

     Copyright:

     Author:

     Date: 2018/4/11 15:59:18

     Description:

         判断是否存在环

 */

 #include <iostream>

 #include <queue>

 #include <vector>

 #include <cstring>

 #include <algorithm>

 using namespace std;

 const int MAXN = 1e+;

 int du[MAXN], n , m, L[MAXN];

 vector<int>  g[MAXN];

 bool topsort() {

     memset(du, , sizeof(du));

     for (int i=; i<n; i++) {

         for (int j=; j<g[i].size(); j++) {

             du[g[i][j]]++;

         }

     }

     int tot = ;

     queue<int> Q;

     for (int i=; i<n; i++) {

         if (!du[i]) {

             Q.push(i);

         }

     }

     while (!Q.empty()) {

         int x = Q.front();

         Q.pop();

         L[tot++] = x;

         for (int j=; j<g[x].size(); j++) {

             int t = g[x][j];

             du[t]--;

             if (!du[t]) {

                 Q.push(t);

             }

         }

     }

     if (tot == n) return ;

     return ;

 }

 int main()

 {

     while (cin>>n>>m && (m || n)) {

         memset(L, , sizeof(L));

         memset(g, , sizeof(g));

         for (int i=; i<m; i++) {

             int a, b;

             cin>>a>>b;

             g[a].push_back(b);

         }

         if (topsort() == ) cout<<"YES"<<endl;

         else cout<<"NO"<<endl;

     }

     return ;

 }