Problem:
Score : 300 points

Problem Statement

You are given an undirected connected graph with N vertices and M edges that does not contain self-loops and double edges.

The i-th edge (1 \leq i \leq M) connects Vertex a_i and Vertex b_i.

An edge whose removal disconnects the graph is called a bridge.

Find the number of the edges that are bridges among the M edges.

Notes

A self-loop is an edge i such that a_i=b_i (1 \leq i \leq M).

Double edges are a pair of edges i,j such that a_i=a_j and b_i=b_j (1 \leq i<j \leq M).

An undirected graph is said to be connected when there exists a path between every pair of vertices.

Constraints

2 \leq N \leq 50

N-1 \leq M \leq min(N(N−1)⁄2,50)

1 \leq a_i<b_i \leq N

The given graph does not contain self-loops and double edges.

The given graph is connected.

Input

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 edges that are bridges among the M edges.

Sample Input 1

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

Sample Output 1

4

The figure below shows the given graph:

The edges shown in red are bridges. There are four of them.

Sample Input 2

3 3
1 2
1 3
2 3

Sample Output 2

0

It is possible that there is no bridge.

Sample Input 3

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

Sample Output 3

5

It is possible that every edge is a bridge.

Solution in Rust (1.15.1) (judge verdict: Wrong Answer)

fn get_line() -> String {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().to_string()
}

fn reads<T>() -> Vec<T>
    where
        T: std::str::FromStr,
        <T as std::str::FromStr>::Err: std::fmt::Debug {
    get_line().split(' ').map(|x| x.parse().unwrap()).collect()
}

fn main() {
    let nm = reads();
    let mut xs = vec![vec![false; nm[0]]; nm[0]];
    let mut is = vec![];

    for _ in 0..nm[1] {
        let ab: Vec<usize> = reads();

        xs[ab[0] - 1][ab[1] - 1] = true;
        xs[ab[1] - 1][ab[0] - 1] = true;

    }

    for i in 0..nm[0] {
        is.push(i);
    }

    is.sort_by(|&lhs, &rhs| {
        let mut l = 0;
        let mut r = 0;
        for j in 0..nm[0] {
            if xs[lhs][j] {
                l += 1;
            }
            if xs[rhs][j] {
                r += 1;
            }
        }

        l.cmp(&r)
    });

    let mut res = 0;
    let mut us = vec![true; nm[0]];
    for i in is {
        let mut c = 0;
        for j in 0..nm[0] {
            if i == j {
                continue;
            }

            if us[j] && xs[i][j] {
                c += 1;
            }
        }

        if c == 1 {
            us[i] = false;
            res += 1;
        }
    }

    println!("{}", res);
}