A11503. Almost Acyclic Graph

You are given a [directed graph](https://en.wikipedia.org/wiki/Directed_graph) consisting of $n$ vertices and $m$ edges (each edge is directed, so it can be traversed in only one direction). You are allowed to remove at most one edge from it.

Can you make this graph [acyclic](https://en.wikipedia.org/wiki/Directed_acyclic_graph) by removing at most one edge from it? A directed graph is called acyclic iff it doesn't contain any cycle (a non-empty path that starts and ends in the same vertex).

The first line contains two integers $n$ and $m$ ( $2<=n<=500$ , $1<=m<=min(n(n-1),100000)$ ) — the number of vertices and the number of edges, respectively.

Then $m$ lines follow. Each line contains two integers $u$ and $v$ denoting a directed edge going from vertex $u$ to vertex $v$ ( $1<=u,v<=n$ , $u≠v$ ). Each ordered pair $(u,v)$ is listed at most once (there is at most one directed edge from $u$ to $v$ ).

If it is possible to make this graph acyclic by removing at most one edge, print YES. Otherwise, print NO.

In the first example you can remove edge , and the graph becomes acyclic.

In the second example you have to remove at least two edges (for example,  and ) in order to make the graph acyclic.