CF977E.Cyclic Components

You are given an undirected graph consisting of $n$ vertices and $m$ edges. Your task is to find the number of connected components which are cycles.

Here are some definitions of graph theory.

An undirected graph consists of two sets: set of nodes (called vertices) and set of edges. Each edge connects a pair of vertices. All edges are bidirectional (i.e. if a vertex $a$ is connected with a vertex $b$, a vertex $b$ is also connected with a vertex $a$). An edge can't connect vertex with itself, there is at most one edge between a pair of vertices.

Two vertices $u$ and $v$ belong to the same connected component if and only if there is at least one path along edges connecting $u$ and $v$.

A connected component is a cycle if and only if its vertices can be reordered in such a way that:

• the first vertex is connected with the second vertex by an edge,
• the second vertex is connected with the third vertex by an edge,
• ...
• the last vertex is connected with the first vertex by an edge,
• all the described edges of a cycle are distinct.

A cycle doesn't contain any other edges except described above. By definition any cycle contains three or more vertices.

There are $6$ connected components, $2$ of them are cycles: $[7, 10, 16]$ and $[5, 11, 9, 15]$.

给你一个包含 $n$ 个顶点和 $m$ 条边的无向图。你的任务是找出其中构成环的连通分量的数量。

以下是一些图论中的定义：

无向图由两个集合组成：节点集（称为顶点）和边集。每条边连接一对顶点。所有边均为双向的（即若顶点 $a$ 与顶点 $b$ 相连，则顶点 $b$ 也与顶点 $a$ 相连）。一条边不能连接一个顶点与其自身，且任意一对顶点之间至多只有一条边。

当且仅当存在至少一条沿边构成的路径连接顶点 $u$ 和 $v$ 时，$u$ 和 $v$ 属于同一连通分量。

一个连通分量是一个环，当且仅当它的顶点可以重新排序，使得满足以下条件：

• 第一个顶点与第二个顶点之间有一条边，
• 第二个顶点与第三个顶点之间有一条边，
• …
• 最后一个顶点与第一个顶点之间有一条边，
• 上述构成环的所有边互不相同。

环中不包含除上述边之外的任何其他边。根据定义，任意环至少包含三个顶点。

图中共有 $6$ 个连通分量，其中 $2$ 个是环：$[7, 10, 16]$ 和 $[5, 11, 9, 15]$。

The first line contains two integer numbers $n$ and $m$ ($1 \le n \le 2 \cdot 10^5$, $0 \le m \le 2 \cdot 10^5$) — number of vertices and edges.

The following $m$ lines contains edges: edge $i$ is given as a pair of vertices $v_i$, $u_i$ ($1 \le v_i, u_i \le n$, $u_i \ne v_i$). There is no multiple edges in the given graph, i.e. for each pair ($v_i, u_i$) there no other pairs ($v_i, u_i$) and ($u_i, v_i$) in the list of edges.

第一行包含两个整数 $n$ 和 $m$（$1 \le n \le 2 \cdot 10^5$，$0 \le m \le 2 \cdot 10^5$）—— 分别表示顶点数和边数。

接下来的 $m$ 行描述各条边：第 $i$ 条边由一对顶点 $v_i$、$u_i$ 给出（$1 \le v_i, u_i \le n$，$u_i \ne v_i$）。给定图中不存在重边，即对于任意一对 $(v_i, u_i)$，在边列表中不会出现另一个相同的对 $(v_i, u_i)$ 或反向对 $(u_i, v_i)$。

Print one integer — the number of connected components which are also cycles.

输出一个整数——即同时也是环的连通分量的数量。

In the first example only component $[3, 4, 5]$ is also a cycle.

The illustration above corresponds to the second example.

在第一个例子中，只有组件 $[3, 4, 5]$ 同时也是一个环。

上面的示意图对应第二个例子。