C - One-stroke Path /

### 問題文

i (1≦i≦M) 番目の辺は頂点 a_i と頂点 b_i を結びます。
ここで、自己ループは a_i = b_i (1≦i≦M) となる辺のことを表します。
また、二重辺は a_i=a_j かつ b_i=b_j (1≦i<j≦M) となる辺のことを表します。

ただし、パスの始点と終点の頂点も訪れたものとみなします。

このとき、図2で表されるパスは条件を満たします。

しかし、図3で表されるパスは条件を満たしません。全ての頂点を訪れていないからです。

また、図4で表されるパスも条件を満たしません。始点が頂点 1 ではないからです。

### 制約

• 2≦N≦8
• 0≦M≦N(N-1)/2
• 1≦a_i<b_i≦N
• 与えられるグラフは自己ループと二重辺を含まない。

### 入力

N M
a_1 b_1
a_2 b_2
:
a_M b_M


### 入力例 1

3 3
1 2
1 3
2 3


### 出力例 1

2


### 入力例 2

7 7
1 3
2 7
3 4
4 5
4 6
5 6
6 7


### 出力例 2

1


このテストケースは問題文の例と同じです。

Score : 300 points

### Problem Statement

You are given an undirected unweighted graph with N vertices and M edges that contains neither self-loops nor double edges.
Here, a self-loop is an edge where a_i = b_i (1≤i≤M), and double edges are two edges where (a_i,b_i)=(a_j,b_j) or (a_i,b_i)=(b_j,a_j) (1≤i<j≤M).
How many different paths start from vertex 1 and visit all the vertices exactly once?
Here, the endpoints of a path are considered visited.

For example, let us assume that the following undirected graph shown in Figure 1 is given.

Figure 1: an example of an undirected graph

The following path shown in Figure 2 satisfies the condition.

Figure 2: an example of a path that satisfies the condition

However, the following path shown in Figure 3 does not satisfy the condition, because it does not visit all the vertices.

Figure 3: an example of a path that does not satisfy the condition

Neither the following path shown in Figure 4, because it does not start from vertex 1.

Figure 4: another example of a path that does not satisfy the condition

### Constraints

• 2≦N≦8
• 0≦M≦N(N-1)/2
• 1≦a_i<b_i≦N
• The given graph contains neither self-loops nor double edges.

### Input

The input is given from Standard Input in the following format:

N M
a_1 b_1
a_2 b_2
:
a_M b_M


### Output

Print the number of the different paths that start from vertex 1 and visit all the vertices exactly once.

### Sample Input 1

3 3
1 2
1 3
2 3


### Sample Output 1

2


The given graph is shown in the following figure:

The following two paths satisfy the condition:

### Sample Input 2

7 7
1 3
2 7
3 4
4 5
4 6
5 6
6 7


### Sample Output 2

1


This test case is the same as the one described in the problem statement.