1 #include<cstdio>
 2 #define N 420000
 3 int head[N],next[N],to[N],rs[N],n,m,a,b,y,num;
 4 int dfs(int x){
 5     for(int i=head[x];i;i=next[i]){
 6         y=to[i];
 7         if(rs[y]==rs[x])
 8             return 0;
 9         if(!rs[y])
10             rs[y]=-rs[x];
11     }
12     return 1;
13 }
14 int main(){
15     scanf("%d%d",&n,&m);
16     if(n==1){
17         printf("NO");
18         return 0;
19     }
20     for(int i=1;i<=m;++i){
21         scanf("%d%d",&a,&b);
22         next[++num]=head[a];
23         to[num]=b;
24         head[a]=num;
25     }
26     rs[1]=1;
27     for(int i=1;i<=n;++i){
28         if(!dfs(i)){
29             printf("NO");
30             return 0;
31         }
32     }
33     printf("YES");
34     return 0;
35 }