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.IOException;
import java.util.Scanner;

/**
 * Created on 2017/10/20 7:19.
 */
public class Main {
    static int N;

    static boolean isReachable(boolean used[], boolean[][] t, int pos, int cutFrom, int cutTo) {
        if (pos == cutTo) {
            return true;
        }
        used[pos] = true;
        for (int i = 0; i < N; i++) {
            if (!used[i] && t[pos][i] && !(pos == cutFrom && i == cutTo || pos == cutTo && i == cutTo) && isReachable(used, t, i, cutFrom, cutTo)) {
                return true;
            }
        }
        return false;
    }

    public static void main(String[] args) throws IOException {
        // BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        Scanner sc = new Scanner(System.in);

        while (sc.hasNextInt()) {
            N = sc.nextInt();
            int M = sc.nextInt();
            boolean[][] t = new boolean[N][N];
            for (int i = 0; i < M; i++) {
                int a = sc.nextInt() - 1;
                int b = sc.nextInt() - 1;
                t[a][b] = t[b][a] = true;
            }
            int ans = 0;
            for (int i = 0; i < N; i++) {
                for (int j = i + 1; j < N; j++) {
                    if (!t[i][j]) {
                        continue;
                    }
                    boolean used[] = new boolean[N];
                    if (!isReachable(used, t, i, i, j)) {
                        ans++;
                    }
                }
            }
            System.out.println(ans);
        }
    }
}