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::str::FromStr;

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

fn get_vec<T : FromStr>() -> Vec<T> {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    return s.trim().split_whitespace().map(|e| e.parse().ok().unwrap()).collect();
}

fn dfs(edges : &Vec<Vec<u32>>, mut visited : &mut Vec<bool>, cur : u32, end : u32) -> bool {
    if cur == end {
        return true;
    }
    if visited[cur as usize] {
        return false;
    }
    visited[cur as usize] = true;
    for next in &edges[cur as usize] {
        let ret = dfs(&edges, &mut visited, *next, end);
        if ret {
            return true;
        }
    }
    return false;
}

fn main() {
    let nm : Vec<usize> = get_vec();
    let n = nm[0];
    let m = nm[1];

    let mut es = Vec::new();
    for _i in 0..m {
        let ab : Vec<u32> = get_vec();
        es.push((ab[0],ab[1]));
    }

    let mut ans = 0;
    for i in 0..m {
        let mut edges : Vec<Vec<u32>> = vec!(Vec::new();n);
        for j in 0..m {
            if i != j {
                edges[(es[j].0-1) as usize].push(es[j].1-1);
                edges[(es[j].1-1) as usize].push(es[j].0-1);
            }
        }
        let mut visited = vec!(false; n);
        if !dfs(&edges, &mut visited, es[i].0-1, es[i].1-1) {
            ans += 1;
        }
    }
    println!("{}", ans);
}