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# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;
using System.Collections;
using System.Collections.Generic;
using System.IO;
using System.Linq;

namespace C
{
    public class Program
    {
        static void Main(string[] args)
        {
            var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
            Console.SetOut(sw);
            Solve();
            Console.Out.Flush();
        }

        public static void Solve()
        {
            var NM = Console.ReadLine().Trim().Split(' ').Select(int.Parse).ToArray();
            var N = NM[0];
            var M = NM[1];
            var graph = new List<int>[N].Select(x => new List<int>()).ToArray();
            for (var i = 0; i < M; i++)
            {
                var ab = Console.ReadLine().Trim().Split(' ').Select(int.Parse).ToArray();
                var a = ab[0] - 1;
                var b = ab[1] - 1;
                graph[a].Add(b);
                graph[b].Add(a);
            }

            var answer = 0;
            for (var i = 0; i < N; i++)
            {
                var ok = true;
                for (var j = 0; j < N; j++)
                {
                    if (graph[j].Count == 1)
                    {
                        var index = graph[j].First();
                        graph[index].Remove(j);
                        graph[j].Clear();
                        answer++;
                        ok = false;
                    }
                }
                if (ok) break;
            }
            Console.WriteLine(answer);
        }
    }
}