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.Text;
using System.Linq;
using System.Collections;
using System.Collections.Generic;
using static System.Console;
using static System.Math;
using System.Numerics;

namespace AtCoder
{
    public class Program
    {
        public static void Main(string[] args)
        {
            new Program().Solve(new ConsoleInput(Console.In, ' '));
        }

        public void Solve(ConsoleInput cin)
        {
            var N = cin.ReadInt;
            var M = cin.ReadInt;

            var lines = new string[M].Select(x => cin.ReadLine.Split(' ').Select(x => int.Parse(x)).ToArray()).ToList();

            var points = new int[N + 1].Select(x => new List<int>()).ToArray();

            foreach (var line in lines)
            {
                points[line[0]].Add(line[1]);
                points[line[1]].Add(line[0]);
            }

            Func<int, int[], bool, bool> CanReachAll = null;

            CanReachAll = (int now, int[] isRooted, bool result) => 
            {
                isRooted[now] = 1;
                if (result == true) return true;
                if (isRooted.Sum() == N) return true;
                foreach (var point in points[now])
                {
                    if (isRooted[point] == 1) continue;
                    result = result || CanReachAll(point,isRooted,result);
                }
                return result;
            };

            var ans = 0;
            foreach (var line in lines)
            {
                points[line[0]].Remove(line[1]);
                points[line[1]].Remove(line[0]);

                if (CanReachAll(1, new int[N + 1], false) == false)
                {
                    // WriteLine("{0} {1}",line[0], line[1]);
                    ans++;
                }

                points[line[0]].Add(line[1]);
                points[line[1]].Add(line[0]);
            }
            WriteLine(ans);
        }

        private BigInteger Lcm(BigInteger a, BigInteger b) 
        {
            return (a * b) / Gcd(a, b);
        }

        /// <summary>
        /// 最大公約数計算
        /// </summary>
        /// <param name="a"></param>
        /// <param name="b"></param>
        /// <returns></returns>
        private BigInteger Gcd(BigInteger a, BigInteger b) 
        {
            if (a < b) return Gcd(b, a);
            
            while (b != 0) 
            {
                var mod = a % b;
                a = b;
                b = mod;
            }
            return a;
        }
    }

    public class ConsoleInput
    {
        private readonly System.IO.TextReader _stream;
        private char _separator = ' ';
        private Queue<string> inputStream;
        public ConsoleInput(System.IO.TextReader stream, char separator = ' ')
        {
            this._separator = separator;
            this._stream = stream;
            inputStream = new Queue<string>();
        }

        public string Read
        {
            get
            {
                if (inputStream.Count != 0) return inputStream.Dequeue();
                string[] tmp = _stream.ReadLine().Split(_separator);
                for (int i = 0; i < tmp.Length; i++)
                    inputStream.Enqueue(tmp[i]);
                return inputStream.Dequeue();
            }
        }

        public string ReadLine { get { return _stream.ReadLine(); } }
        public int ReadInt { get { return int.Parse(Read); } }
        public long ReadLong { get { return long.Parse(Read); } }
        public double ReadDouble { get { return double.Parse(Read); } }
        public string[] ReadStrArray(long N) { var ret = new string[N]; for (long i = 0; i < N; ++i) ret[i] = Read; return ret; }
        public int[] ReadIntArray(long N) { var ret = new int[N]; for (long i = 0; i < N; ++i) ret[i] = ReadInt; return ret; }
        public long[] ReadLongArray(long N) { var ret = new long[N]; for (long i = 0; i < N; ++i) ret[i] = ReadLong; return ret; }
    }

    /// <summary>
    /// Mod計算用クラス
    /// </summary>
    struct ModInt
    {
        const int M = 1000000007;
        public long V;
        public ModInt(long v) { V = (v %= M) < 0 ? v + M : v; }

        public static implicit operator ModInt(long v) => new ModInt(v);
        public static ModInt operator +(ModInt x) => x;
        public static ModInt operator -(ModInt x) => -x.V;
        public static ModInt operator +(ModInt x, ModInt y) => x.V + y.V;
        public static ModInt operator -(ModInt x, ModInt y) => x.V - y.V;
        public static ModInt operator *(ModInt x, ModInt y) => x.V * y.V;
        public static ModInt operator /(ModInt x, ModInt y) => x.V * y.Inv();

        public ModInt Pow(int i) => MPow(V, i);
        public ModInt Inv() => MPow(V, M - 2);
        public override string ToString() => $"{V}";

        static long MPow(long b, int i)
        {
            for (var r = 1L; ; b = b * b % M)
            {
                if (i % 2 > 0) r = r * b % M;
                if ((i /= 2) < 1) return r;
            }
        }

        public static long MFactorial(int n) { for (long x = 1, i = 1; ; x = x * ++i % M) if (i >= n) return x; }
        public static long MNpr(int n, int r)
        {
            if (n < r) return 0;
            for (long x = 1, i = n - r; ; x = x * ++i % M) if (i >= n) return x;
        }
        public static ModInt MNcr(int n, int r) => n < r ? 0 : n - r < r ? MNcr(n, n - r) : (ModInt)MNpr(n, r) / MFactorial(r);
    }
}