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: Wrong Answer)

#include <iostream>
#include <vector>
//#include <string>
//#include <algorithm>
//#include <math.h>
#include <queue>
//#include <stack>
//#include <iomanip>

// sometimes used
//#include <set>
//#include <map>
//#include <numeric>
//#include <list>
//#include <deque>
//#include <unordered_map>

//typedef long long LL;
//typedef long double LD;
using namespace std;
 
//#define MOD 1000000007
//#define MAX 100100
#define NIL -1

vector<vector<int>> c(100);
vector<int> is_visit(100, NIL);

void bfs(int s){
    queue<int> q;
    q.push(s);
    int u;
    while(!q.empty()){
        u=q.front();
        is_visit[u]=1;
        q.pop();
        for(int v=0; v<c[u].size(); v++){
            if(is_visit[c[u][v]]==NIL){
                q.push(c[u][v]);
            }
        }
    }
}

int main(){
    int n;
    int m;
    vector<int> a;
    vector<int> b;
    int tmp_a;
    int tmp_b;
    int ans=0;
    cin >> n >> m;
    for(int i=0; i<m; i++){
        cin >> tmp_a >> tmp_b;
        tmp_a--;
        tmp_b--;
        a.push_back(tmp_a);
        b.push_back(tmp_b);
    }

    for(int i=0; i<m; i++){
        for(int j=0; j<n; j++){
            is_visit[j]=NIL;
        }
        for(int k=0; k<m; k++){
            c[k].clear();
        }
        for(int l=0; l<m; l++){
            if(i==l){
                continue;
            }
            c[a[l]].push_back(b[l]);
            c[b[l]].push_back(a[l]);
        }
        bfs(0);
        bool ok=true;
        for(int x=0; x<n; x++){
            if(is_visit[x]==NIL){
                ok=false;
            }
        }
        if(ok==false){
            ans++;
        }
    }

    cout << ans << endl;

    return 0;
}