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>
// statics
using namespace std;
using int64 = int_fast64_t;
using PAIR = pair<int, int>;
constexpr int INF = 1 << 30;
constexpr int64 LINF = 1LL << 60;
constexpr int MOD = 1e9 + 7;
constexpr int MAX_N = 1e5 + 1;

// init/input
#define int int64
#define INIT ios::sync_with_stdio(false);cin.tie(0);
#define VAR(type, ...) type __VA_ARGS__;MACRO_VAR_Scan(__VA_ARGS__);
template<typename T> void MACRO_VAR_Scan(T &t) {cin>>t;}
template<typename First, typename...Rest> void MACRO_VAR_Scan(First &first, Rest&...rest) {cin>>first;MACRO_VAR_Scan(rest...);}
#define VEC(type, c, n) vector<type> c(n);for(auto &&i:c)cin>>i;

// out
#define OUT(dist) cout<<(dist);
#define FOUT(n, dist) cout<<fixed<<setprecision(n)<<(dist);
#define SP cout<<" ";
#define BR cout<<"\n";
#define debug(x) cerr<<#x<<":"<< (x);BR;

// utility
#define ALL(a) (a).begin(), (a).end()
#define EACH(i, a) for(auto &&i:(a))
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define RFOR(i, a, b) for(int i=(b)-1;i>=0;--i)
#define REP(i, n) for(int i=0;i<(n);++i)
#define RREP(i, n) for(int i=(n)-1;i>=0;--i)

struct UnionFind {
  vector< int > par;
  UnionFind(int n = 1) {
    init(n);
  }

  void init(int n = 1) {
    par.resize(n);
    for(int i = 0; i < n; i++)
      par[i] = -1;
  }
  
  int root(int n) {
    if (par[n] < 0) return n;
    return par[n] = root(par[n]);
  }

  bool merge(int x, int y) {
    x = root(x); y = root(y);
    if (x == y) return false;
    if (par[x] > par[y]) swap(x, y);
    par[x] += par[y];
    par[y] = x;

    return true;
  }

  bool issame(int x, int y) {
    return root(x) == root(y);
  }
};

signed main() {
  INIT;
	VAR(int, n, m);
	vector< int > a(m), b(m);
	REP(i, m) {
		cin >> a[i] >> b[i];
		a[i]--; b[i]--;
	}

	int ans = 0;
	REP(i, n) {
		UnionFind uf(n + 1);
		REP(j, m) {
			if (i == j) continue;
			uf.merge(a[j], b[j]);
		}

		set< int > st;
		REP(i, n) {
			st.insert(uf.root(i));
		}

		if (st.size() > 1) ans++;
	}
	
	OUT(ans)BR;
  
  return 0;
}