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 (1.42.0) (judge verdict: Accepted)

use std::io::Read;
use std::collections::VecDeque;

fn main() {
    let mut buf = String::new();
    std::io::stdin().read_to_string(&mut buf).unwrap();

    let answer = solve(&buf);

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

fn solve(input: &str) -> String {
    let mut iterator = input.split_whitespace();

    let n: usize = iterator.next().unwrap().parse().unwrap();
    let m: usize = iterator.next().unwrap().parse().unwrap();
    let mut edges: Vec<(usize, usize)> = Vec::new();
    let mut graph: Vec<Vec<usize>> = vec![vec![]; n];
    for _ in 0..m {
        let a: usize = iterator.next().unwrap().parse().unwrap();
        let b: usize = iterator.next().unwrap().parse().unwrap();
        edges.push((a - 1, b - 1));
        graph[a - 1].push(b - 1);
        graph[b - 1].push(a - 1);
    }

    let mut ans: usize = 0;
    for edge in edges {
        let mut visited: Vec<bool> = vec![false; n];
        let mut q: VecDeque<usize> = VecDeque::new();
        q.push_back(0);
        visited[0] = true;
        while !q.is_empty() {
            let u: usize = q.pop_front().unwrap();
            for &v in &graph[u] {
                if u == edge.0 && v == edge.1 || u == edge.1 && v == edge.0 {
                    continue;
                }
                if !visited[v] {
                    visited[v] = true;
                    q.push_back(v);
                }
            }
        }
        if visited.iter().any(|it| *it == false) {
            ans += 1;
        }
    }
    ans.to_string()
}