#include <iostream>
#include <cstring>
#include <queue>
using namespace std;
int relation[108];

int find(int person){//查找
    int temp=person;
    while(relation[temp]!=temp)//返回根
        temp=relation[temp];

    int temp2=person,temp3;
    while(relation[temp2]!=temp){//优化
        temp3=relation[temp2];
        relation[temp2]=temp;
        temp2=temp3;
    }
    return temp;
}

void union_(int son,int father){//并集
    int findson=find(son);//找到根
    int findfather=find(father);
    if(findson!=findfather){//根不相同
        relation[findson]=findfather;//合并
    }
}
int main()
{
    int numOfPoints,numOfSections;
    int graph[108][108];
    int in[108],out[108];
    int count;
    queue<int> que;
    while(cin>>numOfPoints>>numOfSections){
        count=0;
        while(!que.empty()){
            que.pop();
        }
        int a,b;
        memset(graph,0,sizeof(graph));
        memset(in,0,sizeof(in));
        memset(out,0,sizeof(out));
        for(int i=1;i<=numOfSections;i++){
            cin>>a>>b;
            out[a]++;
            in[b]++;
            graph[a][b]++;
        }
        for(int i=1;i<=numOfPoints;i++){
            if(in[i]==0){
                count++;
                que.push(i);
            }
        }
        while(!que.empty()){
            int front=que.front();
            que.pop();
            for(int i=1;i<=numOfPoints;i++){
                if(graph[front][i]){
                    in[i]-=graph[front][i];
                    if(in[i]==0){
                        que.push(i);
                        count++;
                    }
                }
            }
        }
        if(count==numOfPoints)
            cout<<"YES"<<endl;
        else
            cout<<"NO"<<endl;
    }
    return 0;
}