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.Generic;
using System.Linq;
using System.Text;

public class AtCoderB
{
    static void Main()
    {
        var sc = new Scanner();

        long n = sc.NextLong();
        long m = sc.NextLong();

        List<long>[] from = new List<long>[n + 1];
        Tuple<long, long>[] edge = new Tuple<long, long>[m];

        for (int i = 0; i <= n; i++)
        {
            from[i] = new List<long>();
        }

        for (int i = 0; i < m; i++)
        {
            long a = sc.NextLong();
            long b = sc.NextLong();
            from[a].Add(b);
            from[b].Add(a);
            edge[i] = Tuple.Create(a, b);
        }

        long answer = 0;
        for (int i = 0; i < m; i++)
        {
            var ng1 = edge[i].Item1;
            var ng2 = edge[i].Item2;

            bool[] can = new bool[n + 1];
            can[0] = true;

            Queue<long> que = new Queue<long>();
            que.Enqueue((long)1);
            while (que.Count > 0)
            {
                var point = que.Dequeue();
                if (can[point])
                {
                    continue;
                }
                can[point] = true;

                foreach (var target in from[point])
                {
                    if ((ng1 == point && ng2 == target) || (ng2 == point && ng1 == target))
                    {
                        continue;
                    }
                    que.Enqueue(target);
                }
            }
            if (can.Where(c => !c).Count() > 0)
            {
                answer++;
            }
        }

        Console.Write(answer);
    }

    class Scanner
    {
        private string[] _line;
        private int _index;
        private const char Separator = ' ';

        public Scanner()
        {
            _line = new string[0];
            _index = 0;
        }

        public string Next()
        {
            while (_index >= _line.Length)
            {
                _line = Console.ReadLine().Split(Separator);
                _index = 0;
            }

            return _line[_index++];
        }

        public int NextInt() => int.Parse(Next());
        public long NextLong() => long.Parse(Next());
        public double NextDouble() => double.Parse(Next());
        public decimal NextDecimal() => decimal.Parse(Next());
        public char NextChar() => Next()[0];
        public char[] NextCharArray() => Next().ToCharArray();

        public string[] Array()
        {
            _line = Console.ReadLine().Split(Separator);
            _index = _line.Length;
            return _line;
        }

        public int[] IntArray() => Array().Select(int.Parse).ToArray();
        public long[] LongArray() => Array().Select(long.Parse).ToArray();
        public double[] DoubleArray() => Array().Select(double.Parse).ToArray();
        public decimal[] DecimalArray() => Array().Select(decimal.Parse).ToArray();
    }
}