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 C# (Mono 4.6.2.0) (judge verdict: Accepted)

//#pragma warning disable
using System.Collections;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System;
using System.Numerics;
using System.Threading.Tasks;
using static System.Math;
using static System.Console;
class Z { static void Main() => new K(); }
class K
{
	int F() => int.Parse(ReadLine());
	long FL() => long.Parse(ReadLine());
	int[] G() => ReadLine().Split(new char[] { ' ', '\t' }, StringSplitOptions.RemoveEmptyEntries).Select(int.Parse).ToArray();
	long[] GL() => ReadLine().Split(new char[] { ' ', '\t' }, StringSplitOptions.RemoveEmptyEntries).Select(long.Parse).ToArray();
	public const int MOD = 1000000007;
	public K()
	{
		SetOut(new StreamWriter(OpenStandardOutput()) { AutoFlush = false });
		Solve();
		Out.Flush();
	}
	int N, M;
	HashSet<int>[] es;
	void Solve()
	{
		var I = G();
		N = I[0];
		M = I[1];
		es = new HashSet<int>[N];
		for (int i = 0; i < N; i++) es[i] = new HashSet<int>();
		var rs = new Tuple<int, int>[M];
		for (int i = 0; i < M; i++)
		{
			I = G();
			int a = I[0] - 1, b = I[1] - 1;
			es[a].Add(b);
			es[b].Add(a);
			rs[i] = new Tuple<int, int>(a, b);
		}
		var s = 0;
		foreach (var e in rs)
		{
			int a = e.Item1, b = e.Item2;
			es[a].Remove(b);
			es[b].Remove(a);
			if (!Connected()) s++;
			es[a].Add(b);
			es[b].Add(a);
		}
		WriteLine(s);
	}
	bool[] used;
	bool Connected()
	{
		used = new bool[N];
		DFS(0);
		foreach (var x in used) if (!x) return false;
		return true;
	}
	void DFS(int u)
	{
		foreach (var v in es[u])
			if (!used[v])
			{
				used[v] = true;
				DFS(v);
			}
	}
}