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 static System.Math;
using static System.Console;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace AtCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            var line = ReadLine().Split(' ').Select(value => int.Parse(value)).ToArray();
            var n = line[0];
            var m = line[1];
            var edges = new List<int[]>();
            for (var i = 0; i < m; i++)
            {
                line = ReadLine().Split(' ').Select(value => int.Parse(value) - 1).ToArray();
                edges.Add(line);
            }

            var count = 0;
            for (var i = 0; i < m; i++)
            {
                var parents = new int[n];
                for (var j = 0; j < n; j++)
                {
                    parents[j] = j;
                }

                for (var j = 0; j < m; j++)
                {
                    if (i == j)
                    {
                        continue;
                    }
                    var edge = edges[j];
                    Unite(parents, edge[0], edge[1]);
                }

                for(var j = 0; j < n; j++)
                {
                    GetRoot(parents, j);
                }

                if (!parents.All(value => value == parents[0]))
                {
                    count++;
                }
            }

            WriteLine(count);
        }

        private static int GetRoot(int[] parents, int node)
        {
            if (parents[node] == node)
            {
                return node;
            }
            return parents[node] = GetRoot(parents, parents[node]);
        }

        private static void Unite(int[] parents, int x, int y)
        {
            x = GetRoot(parents, x);
            y = GetRoot(parents, y);

            if (x == y)
            {
                return;
            }
            parents[x] = y;
        }
    }
}