Problem:
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.

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;

//解説コードを理解してC#で解いたもの
class Progaram
{
    static readonly int NMAX = 8;
    static int N;
    static int M;
    static bool[,] graph = new bool[NMAX, NMAX];

    static int Dfs(int v, bool[] visited)
    {
        var allv = true;
        for (var i = 0; N > i; i++) if (visited[i] == false) allv = false;
        if (allv) return 1;

        var ret = 0;

        for (var i = 0; N > i; i++)
        {
            if (graph[v, i] == false) continue;
            if (visited[i]) continue;
            visited[i] = true;
            ret += Dfs(i, visited);
            visited[i] = false;
        }
        return ret;
    }

    static void Main(string[] args)
    {
        var line = Console.ReadLine().Split(' ')
            .Select(x => int.Parse(x)).ToArray();
        N = line[0]; M = line[1];
        
        for (var i = 0; M > i; i++)
        {
            var l = Console.ReadLine().Split(' ')
                .Select(x => int.Parse(x)).ToArray();
            graph[l[0] - 1, l[1] - 1] = true;
            graph[l[1] - 1, l[0] - 1] = true;
        }
        var visited = new bool[NMAX];
        for (var i = 0; N > i; i++) visited[i] = false;
        visited[0] = true;

        Console.WriteLine(Dfs(0, visited));
    }
}