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: Accepted)

use std::collections::VecDeque;

#[allow(unused_macros)]
macro_rules! get {
    ([$t:ty] ; $n:expr) => {
        (0..$n)
        .map(|_| get!([$t])).collect::<Vec<_>>()
    };
    ($($t:ty),+ ; $n:expr) => {
        (0..$n).map(|_| get!($($t),+)).collect::<Vec<_>>()
    };
    ([$t:ty]) => {
        rl().split_whitespace()
        .map(|x| x.parse().unwrap())
        .collect::<Vec<$t>>()
    };
    ($t:ty) => {
        rl().parse::<$t>().unwrap()
    };
    ($($t:ty),*) => {
        {
            let line =rl();
            let mut iter = line.split_whitespace();
            ($(iter.next().unwrap().parse::<$t>().unwrap()),*)
        }
    };
}
fn rl() -> String {
    let mut line = String::new();
    std::io::stdin().read_line(&mut line).unwrap();
    line.trim().to_string()
}

fn main() {
    let (n, m) = get!(usize, usize);
    let mut edges = Vec::new();
    let mut v = vec![Vec::new(); n];
    for _ in 0..m {
        let (mut a, mut b) = get!(usize, usize);
        a -= 1;
        b -= 1;
        edges.push((a, b));
        v[a].push(b);
        v[b].push(a);
    }
    //eprintln!("{:?}", v);
    //eprintln!("{:?}", edges);
    let mut ans = 0;
    'outer: for i in 0..m {
        let mut visited = vec![false; n];
        let mut q = VecDeque::new();
        q.push_back(0);
        //eprintln!("edges[{}]: {:?}", i, edges[i]);
        while !q.is_empty() {
            let now = q.pop_front().unwrap();
            visited[now] = true;
            for j in 0..v[now].len() {
                let next = v[now][j];

                if (edges[i].0 == now && edges[i].1 == next)
                    || (edges[i].1 == now && edges[i].0 == next)
                {
                    continue;
                }
                //eprintln!("now: {}, next: {}", now, next);
                if visited[v[now][j]] == true {
                    continue;
                }
                q.push_back(v[now][j]);
            }
        }
        //eprintln!("edge: {:?}", edges[i]);
        //eprintln!("visited: {:?}", visited);
        for k in 0..n {
            if visited[k] == false {
                ans += 1;
                continue 'outer;
            }
        }
    }
    println!("{}", ans);
}