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>
#define INF 1e9
#define llINF 1e18
#define MOD 1000000007
#define pb push_back
#define mp make_pair 
#define F first
#define S second
#define ll long long
#define ull unsigned long long
#define vi vector<ll>
#define vvi vector<vi>
#define DBG_N(hoge) cerr<<" "<<(hoge)<<endl;
#define DBG cerr<<"!"<<endl;
#define BITLE(n) (1LL<<((ll)n))
#define BITCNT(n) (__builtin_popcountll(n))
#define SUBS(s,f,t) ((s).substr((f)-1,(t)-(f)+1))
#define ALL(a) (a).begin(),(a).end()
using namespace std;
ll cnt=0;
bool vis[100];
vector<pair<ll,ll>>pll;
void dfs(ll now,ll unused){
  cnt++;
  ll m=pll.size();
  for(int i=0;i<m;i++){
    if(i==unused)continue;
    if(pll[i].F==now){
      if(vis[pll[i].S]==false){
	vis[pll[i].S]=true;
	dfs(pll[i].S,unused);
	//	vis[pll[i].S]=false;
      }
    }else if(pll[i].S==now){
      if(vis[pll[i].F]==false){
	vis[pll[i].F]=true;
	dfs(pll[i].F,unused);
	//vis[pll[i].F]=false;
      }
    }
  }
}
int main(){
  cin.tie(0);
  ios::sync_with_stdio(false);
  ll n,m;cin>>n>>m;
  pll.resize(m);
  ll ans=0;
  for(int i=0;i<m;i++)cin>>pll[i].F>>pll[i].S;
  for(int i=0;i<m;i++){
    cnt=0;
    for(int j=1;j<=50;j++)vis[j]=false;
    vis[1]=true;
    dfs(1,i);
    //cout<<cnt<<" "<<pll[i].F<<" "<<pll[i].S<<endl;
    if(cnt!=n){ans++;}
  }
  cout<<ans<<endl;
  return 0;
}