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)

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

class Program
{
    class P
    {
        public int index;
        public List<int> ads;

        public P(int index)
        {
            this.index = index;
            ads = new List<int>();
        }
    }
    static void Main(string[] args)
    {
        var nm = Array.ConvertAll(ReadLine().Split(' '), int.Parse);
        var ab = new int[nm[1]][];
        for (var i = 0; i < nm[1]; i++)
        {
            var s = Array.ConvertAll(ReadLine().Split(' '), int.Parse);
            ab[i] = new int[2];
            ab[i][0] = s[0];
            ab[i][1] = s[1];
        }
        var result = 0;
        for (var i = 0; i < nm[1]; i++)
        {
            var ps = new P[nm[0]];
            for (var j = 0; j < nm[0]; j++) ps[j] = new P(j);
            for (var j = 0; j < nm[1]; j++)
            {
                if (i == j) continue;
                ps[ab[j][0]-1].ads.Add(ab[j][1]-1);
                ps[ab[j][1]-1].ads.Add(ab[j][0]-1);
            }
            var s = new HashSet<int>();
            iscorr(ps, s, 0);
            if (s.Count != nm[0]) result++;
        }
        WriteLine(result);
    }

    private static void iscorr(P[] ps, HashSet<int> set,int index)
    {
        set.Add(index);
        foreach (var p in ps[index].ads)
        {
            if (!set.Contains(p)) iscorr(ps, set,p); 
        }
    }
}