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;
using static System.Console;

class Program {
    static Boolean[] visited;
    static Boolean[,] con;
    static int N;

    internal static void Main(string[] args) {
        var inp = ReadLine().Split(' ').Select(s => int.Parse(s)).ToList();
        N = inp[0];
        var M = inp[1];
        con = new Boolean[N + 1, N + 1];
        var a = new List<int>();
        var b = new List<int>();
        for (int i = 0; i < M;i++) {
            inp = ReadLine().Split(' ').Select(s => int.Parse(s)).ToList();
            inp[0]--;
            inp[1]--;
            a.Add(inp[0]);
            b.Add(inp[1]);
            con[inp[0], inp[1]] = true;
            con[inp[1], inp[0]] = true;
        }

        int ans = 0;
        for (int i = 0; i < M; i++) {
            Change(a[i], b[i], false);
            visited = new Boolean[N];
            DFS(0);
            if (visited.Any(boo => boo == false)) {
                ans++;
            }
            Change(a[i], b[i], true);
        }
        WriteLine(ans);
    }

    static void DFS(int x) {
        visited[x] = true;
        for (int i = 0; i < N; i++) {
            if (con[x,i] == false) { continue; }
            if (visited[i] == true) { continue; }
            DFS(i);
        }
    }

    static void Change(int a, int b, Boolean boo) {
        con[a, b] = boo;
        con[b, a] = boo;
    }
}