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.IO;
using System.Linq;
using System.Numerics;
using System.Text;
using static System.Console;
using static System.Convert;
using static System.Math;
using static Extentions;

class IO
{
    int idx;
    string[] input = In.ReadToEnd().Split(new[] { " ", "\n", "\r" },
        StringSplitOptions.RemoveEmptyEntries);

    T Get<T>(Func<string, T> parser) => parser(input[idx++]);

    public string S => Get(s => s);
    public char C => Get(char.Parse);
    public int I => Get(int.Parse);
    public long L => Get(long.Parse);
    public double F => Get(double.Parse);
    public decimal D => Get(decimal.Parse);
    public BigInteger B => Get(BigInteger.Parse);

    T[] Gets<T>(int n, Func<string, T> parser)
        => input.Skip((idx += n) - n).Take(n).Select(parser).ToArray();

    public string[] Ss(int n) => Gets(n, s => s);
    public char[] Cs(int n) => Gets(n, char.Parse);
    public int[] Is(int n) => Gets(n, int.Parse);
    public long[] Ls(int n) => Gets(n, long.Parse);
    public double[] Fs(int n) => Gets(n, double.Parse);
    public decimal[] Ds(int n) => Gets(n, decimal.Parse);
    public BigInteger[] Bs(int n) => Gets(n, BigInteger.Parse);

    public void Write<T>(params T[] xs) => WriteLine(string.Join(" ", xs));
    public void Write(params object[] xs) => WriteLine(string.Join(" ", xs));
}

class UnionFind
{
    int[] tree;

    // O(N)
    public int CountGroups => tree.Count(x => x < 0);

    // O(N)
    public UnionFind(int length)
    {
        tree = Enumerable.Repeat(-1, length).ToArray();
    }

    // O(α(N))
    public bool Union(int x, int y)
    {
        x = this.Find(x);
        y = this.Find(y);
        if (x == y) return false;
        if (tree[x] > tree[y]) Swap(ref x, ref y);
        tree[x] += tree[y];
        tree[y] = x;
        return true;
    }

    // O(α(N))
    public int Find(int x)
    {
        if (tree[x] < 0) return x;
        return tree[x] = this.Find(tree[x]);
    }

    // O(α(N))
    public bool Same(int x, int y) => this.Find(x) == this.Find(y);

    // O(α(N))
    public int Count(int x) => -tree[this.Find(x)];
}

static class Extentions
{
    public static void Swap<T>(ref T x, ref T y)
    {
        var tmp = x;
        x = y;
        y = tmp;
    }
}

static class Program
{
    public static void Main()
    {
        var sw = new StreamWriter(OpenStandardOutput()) { NewLine = "\n" };
#if DEBUG
        sw.AutoFlush = true;
#else
        sw.AutoFlush = false;
#endif
        SetOut(sw);
        Solve(new IO());
        Out.Flush();
    }

    static void Solve(IO io)
    {
        var n = io.I;
        var m = io.I;
        var a = new int[m];
        var b = new int[m];

        for (var i = 0; i < m; i++)
        {
            a[i] = io.I - 1;
            b[i] = io.I - 1;
        }

        var ans = 0;

        for (var i = 0; i < m; i++)
        {
            var uf = new UnionFind(n);
            for (var j = 0; j < m; j++) if (j != i) uf.Union(a[j], b[j]);
            if (uf.CountGroups > 1) ans++;
        }

        io.Write(ans);
    }
}