CF915D.Almost Acyclic Graph

You are given a 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 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).

你被给定一个由 $n$ 个顶点和 $m$ 条边组成的有向图（每条边是有方向的，因此只能沿一个方向遍历）。你最多可以删除其中一条边。

你能否通过最多删除一条边，使该图变为无环图？当且仅当一个有向图不包含任何环（即一条非空路径，其起点与终点为同一顶点）时，该图被称为无环图。

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).

第一行包含两个整数 $n$ 和 $m$（$2 \leq n \leq 500$，$1 \leq m \leq \min(n(n-1),\,100000)$），分别表示顶点数和边数。

接下来是 $m$ 行。每行包含两个整数 $u$ 和 $v$，表示一条从顶点 $u$ 指向顶点 $v$ 的有向边（$1 \leq u,\,v \leq n$，且 $u \neq v$）。每个有序对 $(u,\,v)$ 至多出现一次（即从 $u$ 到 $v$ 的有向边至多有一条）。

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

如果可以通过最多删除一条边使该图变为无环图，则输出 YES；否则输出 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.

在第一个例子中，你可以删除边 ，从而使图变为无环图。

在第二个例子中，你至少需要删除两条边（例如 和 ），才能使图变为无环图。