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 <bits/stdc++.h>

#define rep(i, n) for (int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;
const int lim = 50;

int e[300][300];
int v[300];
ll n, m, ans = 0;

void dfs(int u) {
  v[u] = 1;
  rep(i, n) {
    if (!e[u][i] || v[i]) continue;
    dfs(i);
  }
}

int main() {
  cin >> n >> m;
  vector<ll> a(m), b(m);

  rep(i, m) {
    cin >> a[i] >> b[i];
    a[i]--, b[i]--;
    e[a[i]][b[i]] = e[b[i]][a[i]] = 1;
  }

  rep(i, m) {
    e[a[i]][b[i]] = e[b[i]][a[i]] = 0;
    memset(v, 0, sizeof(v));
    dfs(0);
    bool ok = false;
    rep(j, m) if (v[j]!=0) ok = true;
    if (ok) ans++;
    e[a[i]][b[i]] = e[b[i]][a[i]] = 1;
  }
  cout << ans << endl;
  return 0;
}