#include<iostream>

#include<cstdio>

#include<cstring>

#include<algorithm>

using namespace std;

int indegree[1005],queue[1005],head[1005];

struct Edge

{

	int to;

	int next;

} edge[1005];

int main()

{

	int n,m,i,j,k,iq,a,b;

	while(scanf("%d %d",&n,&m)!=EOF&&n&&m)

	{

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

		memset(indegree,0,sizeof(indegree));

		for(i=1;i<=m;i++)

		{

			scanf("%d %d",&a,&b);

			edge[i].to=b;

			edge[i].next=head[a];

			head[a]=i;

			indegree[b]++;

		}

		iq=0;

		for(i=0;i<n;i++)

		{

			if(indegree[i]==0)

			{

				queue[iq++]=i;

			}

		}

		for(i=0;i<iq;i++)

		{

			for(k=head[queue[i]];k!=-1;k=edge[k].next)

			{

				indegree[edge[k].to]--;

				if(indegree[edge[k].to]==0)

				{

					queue[iq++]=edge[k].to;

				}

			}

		}

		if(iq==n) printf("YES\n");

		else

		printf("NO\n");

	}

}