#include<stdio.h>
#include<string.h>
#include<stack>
#include<algorithm>
#define MAX 10010
#define MAXM 100010
using namespace std;
int ans;
int n,m;
int low[MAX],dfn[MAX];
int instack[MAX],scccnt;
int clock;
stack<int>s;
struct node 
{
	int beg,end,next;
}edge[MAXM];
int head[MAX];
void add(int beg,int end)
{
	edge[ans].beg=beg;
	edge[ans].end=end;
	edge[ans].next=head[beg];
	head[beg]=ans++;
}
void init()
{
	memset(head,-1,sizeof(head));
	ans=0;
}
void tarjan(int u,int fa)
{
	int v;
	low[u]=dfn[u]=++clock;
	s.push(u);
	instack[u]=1;
	for(int i=head[u];i!=-1;i=edge[i].next)
	{
		v=edge[i].end;
		if(!dfn[v])
		{
			tarjan(v,u);
			low[u]=min(low[u],low[v]);
		}
		else if(instack[v])
		    low[u]=min(low[u],dfn[v]);
	}
	if(low[u]==dfn[u])
	{
		scccnt++;
		while(1)
		{
			v=s.top();
			s.pop();
			instack[v]=0;
			if(v==u)
			    break;
		}
	}
}
void solve(int l,int r)
{
	memset(instack,0,sizeof(instack));
	memset(low,0,sizeof(low));
	memset(dfn,0,sizeof(dfn));
	scccnt=clock=0;
	for(int i=l;i<=r;i++)
	    if(!dfn[i])
	        tarjan(i,-1);
	if(scccnt==1)
	    printf("Yes\n");
	else 
	    printf("No\n");
}
int main()
{
	int i,j,a,b;
	while(scanf("%d%d",&n,&m),n|m)
	{
		init();
		while(m--)
		{
			scanf("%d%d",&a,&b);
			add(a,b);
		}
		solve(1,n);
	}
	return 0;
}