hdu 3342

#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
");
        else
            printf("NO
");
    }
    return 0;
}