#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>
#include <limits.h>
#include <ctype.h>
#include <string.h>
#include <string>
#include <math.h>
#include <algorithm>
#include <iostream>
#include <queue>
#include <stack>
#include <deque>
#include <vector>
#include <set>
#include <map>
using namespace std;
#define MAXN 600

int head[MAXN];
int into[MAXN];
//int mark[MAXN];

struct node{
    int to;
    int next;
}edge[600];

int main(){
    int n,m;
    int i,j;

    while(~scanf("%d%d",&n,&m)){
        if(n==0 && m==0){
            break;
        }

        memset(into,0,sizeof(into));
        memset(head,-1,sizeof(head));

        int k = 0;
        while(m--){
            scanf("%d%d",&j,&i);
            edge[k].to = i;
            edge[k].next = head[j];
            head[j] = k++;
            into[i]++;
        }
        queue<int>t;
        for(i=0;i<n;i++){
            if(into[i] == 0){
                t.push(i);
            }
        }
        int count = 0;
        while(!t.empty()){
            int u = t.front();
            count++;
            t.pop();
            for(k=head[u];k!=-1;k=edge[k].next){
                if(--into[edge[k].to] == 0){//--的位置注意
                    t.push(edge[k].to);
                }
            }
        }
        if(count == n){
            printf("YES\n");
        }
        else{
            printf("NO\n");
        }
    }

    return 0;
}