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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include<bits/stdc++.h>
using namespace std;
using ll = long long;
const auto INF = (ll)1e9;
using v = vector<int>;
using vv = vector<v>;
auto a = vector<int>(50,0);
auto b = vector<int>(50,0);
auto mm = vector<v>(50,v(50,0));
auto used = vector<int>(50,0);
int n,m;
void dfs(int s){
    used[s] = 1;
    for (int i = 0; i < n; i++)
    {
        if(mm[s][i]!=1) continue;
        if(used[i]==1) continue;
        dfs(i);
    }
}

int main(){
    cin >> n >> m;
    for (int i = 0; i < m; i++)
    {
        cin >>a[i] >> b[i];
        a[i]--;
        b[i]--;
        mm[a[i]][b[i]]=mm[b[i]][a[i]]=1;
    }

    int sum =0;
    for (int i = 0; i < m; i++)
    {
        mm[a[i]][b[i]] = mm[b[i]][a[i]] = 0;
        generate(used.begin(),used.end(),[](){return 0;});
        dfs(0);
      	bool f = false;
        for(int u = 0;u<n;u++){
            if(used[u]==0){
                f= true;
              	break;
            }
        }
      	if(f) sum++;
        mm[a[i]][b[i]] = mm[b[i]][a[i]] = 1;
    }

    cout << sum;
    return 0;
}