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: Runtime Error)

#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < n; i++)
#define rrep(i, n) for (int i = n - 1; i >= 0; i--)
using namespace std;
using Graph = vector<vector<int>>;
#define MOD 1000000007
#define MOD2 998244353
#define INF ((1<<30)-1)
#define LINF (1LL<<60)
#define EPS (1e-10)
typedef long long ll;
typedef pair<ll, ll> P;

bool seen[55];

void dfs(vector<int> g[55], int v){
    seen[v] = true;
    for(auto to : g[v]){
        if (seen[to]) continue;
        dfs(g, to);
    }
}

int main(){
    int n, m, ans = 0;
    cin >> n >> m;
    int a[n], b[n];
    rep(i, m) cin >> a[i] >> b[i];
    rep(i, m){
        vector<int> g[55];
        rep(j, m){
            if (j != i) {
                g[a[j]-1].push_back(b[j]-1);
                g[b[j]-1].push_back(a[j]-1);
            }
        }
        memset(seen, 0, sizeof(seen));
        dfs(g, 0);
        bool flag = true;
        rep(i, n) if (!seen[i]) flag = false;
        if (!flag) ans ++;
    }
    cout << ans << endl;
    return 0;
}