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 Rust (1.15.1) (judge verdict: Accepted)

fn read<T: std::str::FromStr>() -> T {
  let mut s = String::new();
  std::io::stdin().read_line(&mut s).ok();
  s.trim().parse().ok().unwrap()
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
  read::<String>().split_whitespace()
    .map(|e| e.parse().ok().unwrap()).collect()
}

fn read_vec2<T: std::str::FromStr>(n: u32) -> Vec<Vec<T>> {
  (0..n).map(|_| read_vec()).collect()
}

pub trait LexicalPermutation {
  /// Return `true` if the slice was permuted, `false` if it is already
  /// at the last ordered permutation.
  fn next_permutation(&mut self) -> bool;
}
impl<T> LexicalPermutation for [T] where T: Ord {
  /// Original author in Rust: Thomas Backman <serenity@exscape.org>
  fn next_permutation(&mut self) -> bool {
    // These cases only have 1 permutation each, so we can't do anything.
    if self.len() < 2 { return false; }

    // Step 1: Identify the longest, rightmost weakly decreasing part of the vector
    let mut i = self.len() - 1;
    while i > 0 && self[i-1] >= self[i] {
      i -= 1;
    }

    // If that is the entire vector, this is the last-ordered permutation.
    if i == 0 {
      return false;
    }

    // Step 2: Find the rightmost element larger than the pivot (i-1)
    let mut j = self.len() - 1;
    while j >= i && self[j] <= self[i-1]  {
      j -= 1;
    }

    // Step 3: Swap that element with the pivot
    self.swap(j, i-1);

    // Step 4: Reverse the (previously) weakly decreasing part
    self[i..].reverse();

    true
  }
}
fn main() {
  let (n, m) = {
    let tmp: Vec<usize> = read_vec();
    (tmp[0], tmp[1])
  };

  let mut chart: Vec<(usize, usize)> = Vec::new();

  for _ in 0..m {
    let (a, b) = {
      let tmp: Vec<usize> = read_vec();
      (tmp[0], tmp[1])
    };
    chart.push((a, b));
  }

  let mut graph = vec![vec![false; n]; n];

  for (a, b) in chart {
    graph[a-1][b-1] = true;
    graph[b-1][a-1] = true;
  }


  let mut permutations: Vec<usize> = (0..n).collect();
  let mut count = 0;

  loop {
    let mut flag = true;
    for i in 0..n-1 {
      let a = permutations[i];
      let b = permutations[i + 1];
      if !graph[a][b] {
        flag = false;
        break;
      }
    }
    if permutations[0]==0 && flag { count += 1; }
    if !permutations.next_permutation() { break; }
  }
  println!("{}", count);
}