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.Linq;
using System.Collections.Generic;

namespace ABC075
{
    class C
    {
        static bool[] visited;
        static bool[,] connect;
        static int N;
        static int M;
        static void Main(string[] args)
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            N = input[0];
            M = input[1];

            connect = new bool[N, N];
            visited = new bool[N];

            int[] a = new int[M];
            int[] b = new int[M];
            for (int i = 0; i < M; i++)
            {
                input = Console.ReadLine().Split().Select(int.Parse).ToArray();
                a[i] = input[0] - 1;
                b[i] = input[1] - 1;
                connect[a[i], b[i]] = true;
                connect[b[i], a[i]] = true;
            }

            int res = 0;
            for (int i = 0; i < M; i++)
            {
                connect[a[i], b[i]] = false;
                connect[b[i], a[i]] = false;
                for (int j = 0; j < N; j++) visited[j] = false;

                dfs(0);

                bool bridge = false;
                for (int j = 0; j < N; j++)
                {
                    if (visited[j] == false) bridge = true;
                }
                if (bridge) res++;

                connect[a[i], b[i]] = true;
                connect[b[i], a[i]] = true;
            }
            Console.WriteLine(res);
        }

        static void dfs(int n)
        {
            visited[n] = true;
            for (int i = 0; i < N; i++)
            {
                if (connect[n, i] == false) continue;
                if (visited[i] == true) continue;
                dfs(i);
            }
        }
    }
}