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: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;
using static Atcoder;
using System.Threading.Tasks;
using System.ComponentModel;

public class Hello
{
    private const int INF = 1000000007;
    public static void Main()
    {
        int[] NM = IntLine();
        int N = NM[0], M = NM[1];
        List<Node> node = new List<Node>();
        for (int i = 0; i < N; i++) { Node n = new Node(); node.Add(n); }
        Edge[] edge = new Edge[M];
        int[] box;
        for (int i = 0; i < M; i++)
        {
            box = IntLine().Select(v => v - 1).ToArray();
            edge[i] = new Edge(box);
            node[box[0]].relate.Add(box[1]);
            node[box[0]].num = box[0];
            node[box[1]].relate.Add(box[0]);
            node[box[1]].num = box[1];
        }
        
        int count = 0;
        int ans = 0;
        //各Edgeを落として考える
        for (int i = 0; i < M; i++)
        {
            count = 0;
            Queue<Node> q = new Queue<Node>();
            node.ForEach(v => v.visit = false);
            q.Enqueue(node[0]);
            while (q.Count != 0)
            {
                Node n = q.Dequeue();
                if (n.visit) break;
                n.visit = true;
                foreach (int to in n.relate)
                {
                    if ((n.num == edge[i].from && to == edge[i].to) ||
                        (to == edge[i].from && n.num == edge[i].to)) { continue; }
                    if (!node[to].visit) q.Enqueue(node[to]);
                }
                count++;
            }
            if (count == N) ans++;
        }
        Console.WriteLine(M-ans);
    }


}

public class Node
{
    public HashSet<int> relate = new HashSet<int>();

    public bool visit = false;

    public int num;
}

public class Edge
{
    public int from;
    public int to;
    public Edge(int[] inp) { from = inp[0]; to = inp[1]; }
}

public class Atcoder
{
    public static void Swap<T>(ref T a, ref T b)
    {
        var t = a;
        a = b;
        b = t;
    }
    public static int LCM(int a, int b)
    {
        int c = a * b;
        if (a < b) Swap(ref a, ref b);
        int d = a % b;
        while (d != 0)
        {
            a = b;
            b = d;
            d = a % b;
        }
        return c / b;
    }
    public static string StrReverce(string s)
    {
        return new string(s.Reverse().ToArray());
    }
    public static int IntRead()
    {
        return int.Parse(Console.ReadLine());
    }
    public static bool IsPrime(int n)
    {
        if (n < 2) return false;
        if (n == 2) return true;
        if (n % 2 == 0) return false;

        //double sqrtNum = Math.Sqrt(n);
        for (int i = 3; i < n; i += 2)
        {
            if (n % i == 0) return false;
        }
        return true;
    }
    public static string[] SpRead(char c)
    {
        return Console.ReadLine().Split(c);
    }
    public static long[] LSRead()
    {
        return Console.ReadLine().Split(' ').Select(long.Parse).ToArray();
    }
    public static long Combination(int n, int r)
    {
        if (n == r) return 1;
        return
            Enumerable.Range((n - r + 1), r).ToList().Aggregate((now, next) => now * next) /
            Enumerable.Range(1, r).ToList().Aggregate((now, next) => now * next);
    }
    public static long Permutation(int n, int r)
    {
        if (n == r) return 1;
        return
            Enumerable.Range((n - r), r).ToList().Aggregate((now, next) => now * next);
    }
    public static long LongRead()
    {
        return long.Parse(Console.ReadLine());
    }
    public static long[] LongLine()
    {
        return Console.ReadLine().Split(' ').Select(long.Parse).ToArray();
    }
    public static int[] IntLine()
    {
        return Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
    }
}