1 #include<string>
 2 #include<cstdio>
 3 #include<iostream>
 4 #include<vector>
 5 #include<queue>
 6 #include<stack>
 7 #include<algorithm>
 8 #include<cstring>
 9 #include<stdlib.h>
10 #include<string>
11 #include<cmath>
12 using namespace std;
13 #define pb push_back
14 vector<int >p[10010];
15 int in[10010],n,m,deep[10010],num[10010];
16 void init(){
17     memset(in,0,sizeof(in));
18     memset(deep,0,sizeof(deep));
19     memset(num,0,sizeof(num));
20     for(int i=0;i<=n;i++) p[i].clear();
21 }
22 void tuopu(){
23     queue<int >ak_47;
24     int cnt=0;
25     for(int i=0;i<n;i++)
26     if(in[i]==0)
27         ak_47.push(i),deep[i]=1,num[1]++;
28     while(!ak_47.empty()){
29         int pos=ak_47.front();
30         cnt++;
31         for(int i=0;i<p[pos].size();i++){
32             int to=p[pos][i];
33             if(--in[to]==0) ak_47.push(to),deep[to]=deep[pos]+1,num[deep[to] ]++;
34         }
35         ak_47.pop();
36     }
37     if(cnt<n) cout<<"NO"<<endl;
38     else cout<<"YES"<<endl;
39 
40 }
41 int main(){
42     while(cin>>n>>m,n){
43         init();
44         for(int i=1;i<=m;i++){
45             int a,b;
46             scanf("%d%d",&a,&b);
47             in[b]++;
48             p[a].pb(b);
49         }
50         tuopu();
51     }
52 }