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.Text;
using System.Threading.Tasks;

namespace ABC075_C
{
    class Program
    {
        static void Main(string[] args)
        {
            var nm = Console.ReadLine().Split().Select(int.Parse).ToArray();
            var n = nm[0];
            var m = nm[1];

            var edges = new List<int>[n+1];
            for (int i = 1; i <= n; i++)
            {
                edges[i] = new List<int>();
            }

            for (int i = 0; i < m; i++)
            {
                var ab = Console.ReadLine().Split().Select(int.Parse).ToArray();
                var a = ab[0];
                var b = ab[1];

                edges[a].Add(b);
                edges[b].Add(a);
            }

            var ans = 0;
            while (true)
            {
                var deleted = false;
                for (int i = 1; i <= n; i++)
                {
                    if (edges[i].Count ==1)
                    {
                        edges[edges[i][0]].Remove(i);
                        edges[i] = new List<int>();
                        ans++;
                        deleted = true;
                    }
                }

                if (!deleted) break;
            }

            Console.WriteLine(ans);
        }
    }
}