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)

#pragma GCC optimize ("O3")
#include <iostream>
#include <iomanip>
#include <istream>
#include <ostream>
#include <sstream>
#include <iterator>
#include <vector>
#include <algorithm>
#include <queue>
#include <deque>
#include <list>
#include <stack>
#include <map>
#include <unordered_map>
#include <set>
#include <utility>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <string>
#include <ctime>
#include <cctype>
#include <cstdlib>
#define IINF 10e8
#define INF 1<<30
#define MOD 1000000007
#define mod 1000000007
#define rep(i, a, n) for (ll i = a; i < (ll)(n); i++)
#define Endl endl
#define fi first
#define se second
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define mmax(x,y)(x>y?x:y)
#define mmin(x,y)(x<y?x:y)
#define chmax(x,y) x=mmax(x,y)
#define chmin(x,y) x=mmin(x,y)
#define all(x) (x).begin(),(x).end()
#define siz(x) (ll)(x).size()
#define PI acos(-1.0)
using namespace std;
//using Int=int_fast64_t;
typedef long long int ll;
typedef long long int LL;
typedef pair<ll,ll>Pll;
typedef pair<int,int>Pin;
template<class T> using V=vector<T>;
long long GCD(long long a, long long b) { return b ? GCD(b, a%b) : a; }
long long LCM(long long a, long long b)  {return a/GCD(a,b)*b;}
int dx[4]={-1,0,1,0};
int dy[4]={0,-1,0,1};
int ddx[8]={-1,0,1,0,1,1,-1,-1};
int ddy[8]={0,-1,0,1,1,-1,1,-1};
char dir[4]={'u','l','d','r'};
ll cmp(pair<ll,ll>a,pair<ll,ll> b){
        if(a.se!=b.se)
        return a.se<b.se;
        else
        return a.fi<b.fi;
}
//---------------------------------------------------------------------------
int n,m;
int a[300][300];
void warshall_floyd(){
    for (int k = 0; k < n; k++){       // 経由する頂点
    for (int i = 0; i < n; i++) {    // 始点
        if(a[i][k]==IINF)continue;
        for (int j = 0; j < n; j++) {  // 終点
        if(a[k][j]==IINF)continue;
        chmin(a[i][j], a[i][k] + a[k][j]);
        }
    }
    }
}
//---------------------------------------------------------------------------
int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    //-------------------------------
    cin>>n>>m;
    vector<int>aa(m),bb(m);
    for(int i=0;i<m;i++){cin>>aa[i]>>bb[i];aa[i]--;bb[i]--;}
    int ans=0;
    for(int i=0;i<m;i++){
        for(int x=0;x<n;x++){
            for(int y=0;y<n;y++){
                a[x][y]=IINF;
            }
        }
        for(int j=0;j<m;j++){
            if(i==j)continue;
            else{
                a[aa[j]][bb[j]]=1;
                a[bb[j]][aa[j]]=1;
            }
        }
        warshall_floyd();
          
   
        for(int j=1;j<n;j++){
            if(a[0][j]==IINF){
                ans++;
                break;
            }
        }
    }
    cout<<ans<<endl;
    //-------------------------------
    return 0;
}
//---------------------------------------------------------------------------
/*
int n;cin>>n;
vector<int>a(n);
for(int i=0;i<n;i++)
    cin>>a[i];
*/