#include<stdio.h>
#include<string.h>
#define N 200
int map[N][N],n,m,indegree[N];
int judge() {
int k,i,j;
for(k=0;k<n;k++) {
for(i=0;i<n;i++) {
if(indegree[i])continue;
indegree[i]--;
for(j=0;j<n;j++)
if(map[i][j])
indegree[j]--;
break;
}
if(i==n)return 0;
}
return 1;
}
int main() {
int i,j,k,a,b;
while(scanf("%d%d",&n,&m)!=EOF) {
if(n==0)
return 0;
memset(map,0,sizeof(map));
memset(indegree,0,sizeof(indegree));
while(m--) {
scanf("%d%d",&a,&b);
if(map[a][b]==0) {
map[a][b]=1;
indegree[b]++;
}
}
k=judge();
if(k==1)
printf("YES\n");
else
printf("NO\n");
}
return 0;
}