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 Ruby (2.3.3) (judge verdict: Accepted)

class Solve
  def initialize
    n, m = gets.split.map(&:to_i)
    o = STDIN.read.lines.map{ |l| l.split.map(&:to_i) }

    ans = 0
    m.times do |i|
      g = {}
      tmp = o.dup
      tmp.delete_at(i)
      tmp.each do |a, b|
        g[a] ? (g[a] << b) : (g[a] = [b])
        g[b] ? (g[b] << a) : (g[b] = [a])
      end

      s = Search.new(g)
      s.dfs(1)
      ans += 1 if s.check(n)
    end
    puts ans
  end

  class Search
    def initialize(g)
      @c = {}
      @g = g.dup
    end

    def dfs(v)
      @c[v] = 1
      @g[v].each { |i| dfs(i) unless @c[i]} if @g[v]
    end

    def check(n)
      @c.length != n
    end
  end
end
Solve.new