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>
using namespace std;

#define each(i, c) for (auto& i : c)
#define mkp(a, b) make_pair(a, b)

typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll, ll> Pll;
const ll MOD = 1e9+7;

template<typename P, typename Q> ostream& operator << (ostream& os, pair<P, Q> p) { os << "(" << p.first << ": " << p.second << ")"; return os; }
template<typename T> ostream& operator << (ostream& os, vector<T> v) { os << "("; each (i, v) os << i << ", "; os << ")"; return os; }
template<typename K, typename V> ostream& operator << (ostream& os, map<K, V> m) { os << "{"; each (i, m) os << i << ", "; os << "}"; return os; }

int main() {
  ll n, m;
  cin >> n >> m;

  vector<ll> a(m);
  vector<ll> b(m);
  for (ll i = 0; i < m; ++i) {
    cin >> a[i] >> b[i];
    a[i]--;
    b[i]--;
  }

  ll ans = 0;
  for (ll i = 0; i < m; ++i) {
    vector<vector<ll>> g(n);
    for (ll j = 0; j < m; ++j) {
      if (i == j) continue;
      g[a[j]].push_back(b[j]);
      g[b[j]].push_back(a[j]);
    }

    function<void(ll, vector<bool>&)> solve;
    solve = [&](ll pos, vector<bool> &seen) {
      seen[pos] = true;

      each (v, g[pos]) {
        if (seen[v]) continue;
        solve(v, seen);
      }
    };

    vector<bool> seen(n);
    solve(0, seen);

    ll cnt = 0;
    for (ll i = 0; i < seen.size(); ++i) if (seen[i]) ++cnt;

    if (cnt == n) continue;
    ++ans;
  }

  cout << ans << endl;
  
  return 0;
}