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;
use std::collections::HashSet;

fn read_line() -> String {
    let mut s = String::new();
    io::stdin().read_line(&mut s).unwrap();
    s.trim().to_owned()
}

fn read_pair() -> (i32, i32) {
    let s = read_line();
    let mut it = s.split_whitespace().map(|s| s.parse().unwrap());
    let h = it.next().unwrap();
    let w = it.next().unwrap();
    (h, w)
}

struct UnionFind {
    parent: Vec<usize>,
}

impl UnionFind {
    fn new(n: usize) -> UnionFind {
        UnionFind {
            parent: (0..n).collect(),
        }
    }

    fn find(&mut self, i: usize) -> usize {
        if self.parent[i] == i {
            return i;
        }

        let p = self.parent[i];
        let p = self.find(p);
        self.parent[i] = p;
        p
    }

    fn unite(&mut self, i: usize, j: usize) {
        let pi = self.find(i);
        let pj = self.find(j);
        self.parent[pi] = pj;
    }
}

fn main() {
    let (n, m) = read_pair();
    let vs: Vec<(i32, i32)> = (0..m).map(|_| read_pair()).collect();

    let mut ans = 0;

    for i in 0..vs.len() {
        let mut uf = UnionFind::new(n as usize);
        for j in 0..vs.len() {
            if i != j {
                uf.unite((vs[j].0 - 1) as usize, (vs[j].1 - 1) as usize);
            }
        }

        let mut ss = HashSet::new();
        for j in 0..n {
            ss.insert(uf.find(j as usize));
        }

        if ss.len() != 1 {
            ans += 1;
        }
    }

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