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 int long long
#define all(v) (v).begin(), (v).end()
#define resz(v, ...) (v).clear(), (v).resize(__VA_ARGS__)
#define reps(i, m, n) for(int i = (int)(m); i < (int)(n); i++)
#define rep(i, n) reps(i, 0, n)

template<class T1, class T2> void chmin(T1 &a, T2 b){if(a>b)a=b;}
template<class T1, class T2> void chmax(T1 &a, T2 b){if(a<b)a=b;}

typedef pair<int, int> Pi;
typedef tuple<int, int, int> Ti;
typedef vector<int> vint;

const int inf = 1LL << 55;
const int mod = 1e9 + 7;

struct UnionFind
{
  vector<int> data;
  UnionFind(){}
  UnionFind(int sz):data(sz, -1){};
  int size(int x) { return -data[find(x)]; }
  int find(int x) {
    if(data[x] < 0) return x;
    return data[x] = find(data[x]);
  }
  bool same(int x, int y) {
    return find(x) == find(y);
  }
  void unite(int x, int y)
  {
    x = find(x), y = find(y);
    if(x == y) return;
    if(data[x] < data[y]) swap(x, y);
    data[x] += data[y]; data[y] = x;
    return;
  }
};


signed main()
{
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  cout << fixed << setprecision(12);

  int n, m;
  cin >> n >> m;
  vector<Pi> edge;
  rep(i, m) {
    int a, b;
    cin >> a >> b;
    --a, --b;
    edge.emplace_back(a, b);
  }
  int ans = 0;
  rep(i, m) {
    UnionFind uf(n);
    rep(j, m) {
      if(i == j) continue;
      int u, v;
      tie(u, v) = edge[j];
      uf.unite(u, v);
    }
    if(uf.size(0) != n) ans++;
  }
  cout << ans << endl;

  return 0;
}