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<iostream>
#include<cstdio>
#include<string>
#include<vector>
#include<set>
#include<map>
#include<list>
#include<stack>
#include<queue>
#include<deque>
#include<functional>
#include<utility>
#include<algorithm>
#include<sstream>
#include<cstring>
#include<cmath>
#include<cctype>
#include<exception>
#include<climits>
#include<cfloat>
#include<iomanip>
#include<bitset>
#include<streambuf>
#include<cstdlib>
#include<ctime>
//#include"functions.h"
//#include"highprecision.h"
#define pb push_back
#define pf push_front
#define ppb pop_back
#define mp make_pair
#define np next_permutation
#define fi first
#define sc second
#define vi vector<int>
#define vll vector<long long>
#define vvi vector<vector<int> >
#define vpi vector<pair<int,int> >
#define pii pair<int,int>
#define pll pair<long long,long long>
#define pff pair<float,float>
#define pdd pair<double,double>
#define psi pair<string,int>
#define pis pair<int,string>
#define ppi pair<pii,int>
#define pdi pair<pii,pii>
#define LN 84757
#define INF 1e9
#define LLINF 1e18
#define MAXINT 0x7fffffff
#define MAXLL 0x7fffffffffffffff
#define EPS 1e-5
#define DEPS 1e-9
#define LDEPS 1e-13
#define MOD 1000000007
#define E 2.7182818284
#define PI 3.1415926535
#define rep(a,b) for(int a=0;a<b;a++)
#define forn(a,n) for(int a=1;a<=n;a++)
#define repn(a,n,p) for(int a=0;a<n;a+=p)
#define foreach(a,b) for(__typeof(b.begin())a=b.begin();a!=b.end();a++)
#define fillemp(a) memset(a,0,sizeof(a))
#define fillinf(a) memset(a,0xff,sizeof(a))
#define frestream freopen("input.txt","r",stdin);freopen("output.txt","w",stdout);
#define stdf(a,b) freopen(a,"r",stdin);freopen(b,"w",stdout);
#define inputf(a) freopen(a,"r",stdin);
#define outputf(a) freopen(a,"w",stdout);
#define sz(a) a.size()
#define all(a) a.begin(),a.end()
#define MOD 1000000007
typedef long long ll;
typedef long double ld;
using namespace std;

int n,m,x,cnt;
bool b[60];
vector<pair<int,int> > v[60];
void dfs(int i){
    b[i]=1;
    cnt++;
    for(int j=0;j<v[i].size();j++){
        if(v[i][j].second==x||b[v[i][j].first]){
            continue;
        }
        dfs(v[i][j].first);
    }
}
int main() {
    int ans=0;
    cin>>n>>m;
    for(int i=0;i<m;i++){
        int a,b;
        cin>>a>>b;
        v[a].push_back(make_pair(b,i));
        v[b].push_back(make_pair(a,i));
    }
    for(int i=0;i<m;i++){
        x=i;
        memset(b,0,sizeof(b));
        cnt=0;
        dfs(1);
        if(cnt!=n)ans++;
    }
    cout<<ans<<endl;
    return 0;
}