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::io::prelude::*;
use std::io::stdin;
use std::str::FromStr;

fn main() {
    let n: usize = read();
    let m: usize = read();
    let a: Vec<_> = (0..m)
        .map(|_| (read::<usize>() - 1, read::<usize>() - 1))
        .collect();

    let mut graph = [[false; 50]; 50];
    for i in 0..m {
        graph[a[i].0][a[i].1] = true;
        graph[a[i].1][a[i].0] = true;
    }
    let mut ans = 0;
    for i in 0..m {
        graph[a[i].0][a[i].1] = false;
        graph[a[i].1][a[i].0] = false;

        let mut visited = [false; 50];
        dfs(0, n, &mut graph, &mut visited);
        if visited.iter().take(n).any(|&x| !x) {
            ans += 1;
        }
        graph[a[i].0][a[i].1] = true;
        graph[a[i].1][a[i].0] = true;
    }

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

#[allow(dead_code)]
fn read<T: FromStr>() -> T {
    let stdin = stdin();
    let token: String = stdin
        .lock()
        .bytes()
        .map(|b| b.unwrap() as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect();
    token.parse().ok().unwrap()
}

fn dfs(v: usize, n: usize, graph: &mut [[bool; 50]; 50], visited: &mut [bool]) {
    visited[v] = true;

    for v2 in 0..n {
        if !graph[v][v2] {
            continue;
        }
        if visited[v2] {
            continue;
        }
        dfs(v2, n, graph, visited);
    }
}