#include<stdio.h>
#include<string.h>
int d[15],map[15][15],vis[15];
int main()
{
    int i,j,k,f,n,m,u,v;
    while(~scanf("%d%d",&n,&m))
    {
        memset(d,0,sizeof(d));
        memset(map,0,sizeof(map));
        memset(vis,0,sizeof(vis));
        for(i=1; i<=m; i++)
        {
            scanf("%d%d",&u,&v);
            map[u][v]=1;
            d[v]++;
        }
        for(i=1; i<=n; i++)
        {
            f=0;
            for(j=1; j<=n; j++)
            {
                if(d[j]==0&&vis[j]==0)
                {
                    vis[j]=1;
                    for(k=1; k<=n; k++)
                    if(map[j][k])
                    d[k]--;
                    f=1;
                    break;//找到一个入度为0的就结束
                }
            }
            if(f==0)//如果节点未输出完，而图中就没有了入度为0的节点，则说明有环，不合法。
            break;
        }
        if(f==0)
        printf("NO\n");
        else
        printf("YES\n");
    }
}