#include<cstdio>
#include<cstring>
using namespace std;
const int maxn=(1e4)+5,maxE=(2e4)+5;
const char ans[2][10]={{"NO"},{"YES"}};
int n,m,p[maxn],tot,lnk[maxn],nxt[maxE],son[maxE],T,vis[maxn];
char gt(){
    static char buf[100000],*p1=buf,*p2=buf;
    return p1==p2&&(p2=(p1=buf)+fread(buf,1,100000,stdin),p1==p2)?EOF:*p1++;
}
int read(){
    int ret=0;char ch=gt();
    while(ch<'0'||ch>'9'){if(ch==EOF) return -1;ch=gt();}
    while(ch>='0'&&ch<='9') ret=ret*10+ch-'0',ch=gt();
    return ret;
}
void add_e(int x,int y){son[++tot]=y,nxt[tot]=lnk[x],lnk[x]=tot;}
bool DFS(int x){
    for(int j=lnk[x];j;j=nxt[j])if(vis[son[j]]<T){
        vis[son[j]]=T;
        if(!p[son[j]]||DFS(p[son[j]])){p[son[j]]=x;return 1;}
    }
    return 0;
}
bool check(){
    memset(vis,0,sizeof vis),memset(p,0,sizeof p),T=1;
    for(int i=1;i<=n;i++,T++) if(!DFS(i)) return 0;
    return 1;
}
int main(){
    for(n=read(),m=read();n>=0&&m>=0;n=read(),m=read()){
        memset(lnk,0,sizeof lnk),tot=0;
        for(int i=1;i<=m;i++){
            int x=read(),y=read();
            add_e(x,y);
        }
        printf("%s\n",ans[check()]);
    }
    return 0;
}

#include<cstdio>
#include<cctype>
#include<cstring>
#define maxn 10002
using namespace std;
int n,m,head[maxn],tot,to[maxn<<1],nex[maxn<<1],p[maxn];
bool vis[maxn];

bool dfs(int x){
    for(int i=head[x];i;i=nex[i])
      if(!vis[to[i]]){
        vis[to[i]]=1;
        if(!p[to[i]]||dfs(p[to[i]])){
            p[to[i]]=x;return 1;
        }
    }
    return 0;
}

bool check(){
    tot=1;
    for(int i=1;i<=n;i++,tot++){memset(vis,0,sizeof(vis));if(!dfs(i))return 0;}
    return 1;
}

int main(){
    while(scanf("%d%d",&n,&m)==2){
        memset(head,0,sizeof(head));
        memset(p,0,sizeof(p));
        for(int i=1;i<=m;i++){
            int x,y;scanf("%d%d",&x,&y);
            to[i]=y;nex[i]=head[x];head[x]=i;
        }
        if(!check())printf("NO\n");else printf("YES\n");
    }
}