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.util.Scanner;

public class Main {
	Scanner scan;
	int n;
	int m;

	int[] a;
	int[] b;

	boolean[] judge;
	int count;
	boolean[][] connect;

	public static void main(String args[]){
		new Main().compute();
	}
	void compute() {
		scan=new Scanner(System.in);

		n=scan.nextInt();
		m=scan.nextInt();

		a=new int[m];
		b=new int[m];

		for(int i=0;i<m;i++) {
			a[i]=scan.nextInt();
			b[i]=scan.nextInt();
		}
		judge=new boolean[n];
		count=0;
		connect=new boolean[n][n];


		for(int i=0;i<m;i++) {
			connect[a[i]-1][b[i]-1]=true;
			connect[b[i]-1][a[i]-1]=true;
		}

		for(int i=0;i<m;i++) {
			for(int j=0;j<n;j++) {
				judge[j]=false;
			}
			connect[a[i]-1][b[i]-1]=false;
			connect[b[i]-1][a[i]-1]=false;
			dfs(0);
			connect[a[i]-1][b[i]-1]=true;
			connect[b[i]-1][a[i]-1]=true;
			for(int j=0;j<n;j++) {
				if(!judge[j]) {
					count++;
					break;
				}
			}
		}
		System.out.println(count);
	}
	void dfs(int x) {
		judge[x]=true;
		for(int i=0;i<n;i++) {
			if((connect[x][i])&&(!judge[i])) {
				dfs(i);
			}
		}
	}
}