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 Python (3.4.3) (judge verdict: Accepted)

import sys 
from copy import deepcopy
from collections import deque
sys.setrecursionlimit(10**6)
def LI(): return list(map(int,sys.stdin.readline().rstrip().split()))

def solve(table,ava,now):
    if not 1 in ava:
        global ans
        ans += 1
        return
    global N
    for i in range(N):
        if table[now][i] and ava[i]:
            bt,ba = deepcopy(table),deepcopy(ava)
            bt[now][i] = bt[i][now] = ba[i] = 0
            solve(bt,ba,i)
    return

if __name__=='__main__':
    tmp = deque(LI())
    N,M = tmp.popleft(),tmp.popleft()
    table = [[0]*N for _ in range(N)]
    for i in range(M):
        tmp = deque(LI())
        a,b = tmp.popleft()-1,tmp.popleft()-1
        table[a][b] = table[b][a] = 1
    ans = 0
    ava = [1]*N
    ava[0] = 0
    solve(table,ava,0)
    print(ans)