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++14 (Clang 3.8.0) (judge verdict: Accepted)

#include <iostream>
#include <vector>
using namespace std;
using Graph = vector<vector<int> >;
Graph G;
void dfs(int v, vector<bool> &seen, int &res) {
    bool end = true;
    for (int i = 0; i < seen.size(); ++i) if (!seen[i] && i != v) end = false;
    if (end) {
        ++res;
        return;
    }
    seen[v] = true;
    for (auto nv : G[v]) {
        if (seen[nv]) continue;
        dfs(nv, seen, res);
    }
    seen[v] = false;
}
int main() {
    int N, M; cin >> N >> M;
    G.assign(N, vector<int>());
    for (int i = 0; i < M; ++i) {
        int a, b; cin >> a >> b; --a, --b;
        G[a].push_back(b);
        G[b].push_back(a);
    }
    vector<bool> seen(N, false);
    int res = 0;
    dfs(0, seen, res);
    cout << res << endl;
}