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)

//C - Bridge: 
//Assunto: 
#include<bits/stdc++.h>
#define fi first
#define se second
#define pb push_back
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define loop(i, a, b) for(int i = a; i < b; i++)
#define loopBack(i, a, b) for(int i = a; i >= b; i--)
#define INF INT_MAX
#define LINF LLONG_MAX
#define DINF DBL_MAX
#define MAX 100050
#define MOD 1000000007
using namespace std;
typedef long long int ll;
typedef pair<int, int> ii;

bool vis[55];

void dfs(int no, int i, int j, vector<int> adj[]) {
    if(vis[no]) return;
    vis[no] = true;
    loop(k, 0, adj[no].size()) {
        if(no == i and k == j) continue;
        else dfs(adj[no][k], i, j, adj);
    }
}

int main() {
    ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    //freopen("in.txt", "r", stdin);
    //freopen("out.txt", "w", stdout);
    int total;
    int n, m, x, y, ans = 0;
    cin >> n >> m;
    vector<int> adj[n+1];
    loop(i, 0, m) {
        cin >> x >> y;
        adj[x].pb(y); adj[y].pb(x);
    }
    loop(i, 1, n+1) {
        loop(j, 0, adj[i].size()) {
            memset(vis, false, sizeof(vis));
            dfs(1, i, j, adj);
            total = 0;
            loop(i, 1, n+1) if(vis[i]) total++;
            //cout << total  << " " << n << "\n";
            if(total != n) ans++;
        }
    }
    cout << ans << "\n";
}