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: Runtime Error)

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()
}

fn main (){
    let nm: Vec<i32> = read_vec();
    let edge: Vec<Vec<i32>> = read_vec2(nm[1] as u32);
    //let ans = hoge(&node,&nm[0],&nm[1]);
    let mut graph = vec![vec![false;nm[0] as usize];nm[0] as usize];
    let mut visited = vec![false;nm[0] as usize];
    for i in 0..edge.len() {
        graph[(edge[i][0] - 1) as usize][(edge[i][1] -1 ) as usize] = true;
        graph[(edge[i][1] -1 ) as usize][(edge[i][0] - 1) as usize] = true;
        println!("{:?}",graph[i]);
    }
    let mut ans = 0;
    for i in 0..nm[1] {
        graph[(edge[i as usize][0] - 1) as usize][(edge[i as usize][1] -1 ) as usize] = false;
        graph[(edge[i as usize][1] - 1 ) as usize][(edge[i as usize][0] - 1) as usize] = false;

        for j in 0..nm[0] { visited[j as usize] = false }
        dfs(0,&mut visited,&graph);
        let mut bridge = false;
        for j in 0..nm[0] {
            if visited[j as usize] == false {
                bridge = true;
            }
        }
        if bridge {
            ans += 1
        }
        graph[(edge[i as usize][0] - 1) as usize][(edge[i as usize][1] -1 ) as usize] = true;
        graph[(edge[i as usize][1] -1 ) as usize][(edge[i as usize][0] - 1) as usize] = true;
    }
    print!("{}",ans);
}
fn dfs(v:i32 ,visited:&mut Vec<bool>,graph:&Vec<Vec<bool>>) {
    visited[v as usize] = true;
    for v2 in 0..visited.len() {
        if graph[v as usize][v2 as usize] == false {
            continue;
        }
        if visited[v2 as usize] == true{
            continue;
        }
        dfs(v2 as i32,visited,graph);
    }
}