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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.io.*;
import java.util.*;
import static java.lang.Math.*;

public class Main {
    public static void main(String[] args) {
        Main main = new Main();
        main.solve(args);
    }

    boolean[] check;
    boolean[] used;
    List<List<Integer> > list = new ArrayList<>();
    public void solve(String[] args) {
        MyScanner scanner = new MyScanner();
        int N = scanner.nextInt();
        int M = scanner.nextInt();
        for (int i = 0; i < M; i++) {
            List<Integer> p = new ArrayList<>();
            p.add(scanner.nextInt() -1);
            p.add(scanner.nextInt() -1);
            list.add(p);
        }

        int ret = 0;
        for (int i = 0; i < M; i++) {
            check = new boolean[N];
            used = new boolean[M];
            dfs(0, i);
            boolean f = true;
            for (int j = 0; j < N; j++) {
                if (!check[j]) {
                    f = false;
                }
            }
            if (!f) {
                ret++;
            }
        }
        System.out.println(ret);
    }

    private void dfs(int index, int ignore) {
        check[index] = true;
        for (int i = 0; i < list.size(); i++) {
            if (i == ignore || used[i]) {
                continue;
            }
            List<Integer> p = list.get(i);
            if (p.get(0) == index) {
                used[i] = true;
                dfs(p.get(1), ignore);
            }
            if (p.get(1) == index) {
                used[i] = true;
                dfs(p.get(0), ignore);
            }
        }
    }

    private class MyScanner {
        String[] s;
        int i;
        BufferedReader br;
        String reg = " ";

        MyScanner () {
            s = new String[0];
            i = 0;
            br = new BufferedReader(new InputStreamReader(System.in));
        }

        public String next() {
            try {
                if (i < s.length) return s[i++];
                String line = br.readLine();
                while (line.equals("")) {
                    line = br.readLine();
                }
                s = line.split(reg, 0);
                i = 0;
                return s[i++];
            } catch (Exception e) {
                e.printStackTrace();
            }
            return null;
        }

        public int nextInt() {
            try {
                return Integer.parseInt(next());
            } catch (Exception e) {
                e.printStackTrace();
            }
            return -1;
        }

        public double nextDouble() {
            try {
                return Double.parseDouble(next());
            } catch (Exception e) {
                e.printStackTrace();
            }
            return -1;
        }

        public long nextLong()  {
            try {
                return Long.parseLong(next());
            } catch (Exception e) {
                e.printStackTrace();
            }
            return -1;
        }
    }
}