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 Go (1.6) (judge verdict: Accepted)

package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

var (
	graph [][]bool
	seen  []bool
)

func dfs(now int) {
	if seen[now] {
		return
	}
	seen[now] = true

	for next, b := range graph[now] {
		if !b {
			continue
		}

		dfs(next)
	}
}

func solve() {
	n, m := getInt(), getInt()
	as, bs := make([]int, m), make([]int, m)
	for i := 0; i < m; i++ {
		as[i], bs[i] = getInt()-1, getInt()-1
	}
	graph = make([][]bool, n)
	for i := range graph {
		graph[i] = make([]bool, n)
	}
	for i := 0; i < m; i++ {
		graph[as[i]][bs[i]] = true
		graph[bs[i]][as[i]] = true
	}
	seen = make([]bool, n)

	ans := 0
	for i := 0; i < m; i++ {
		graph[as[i]][bs[i]] = false
		graph[bs[i]][as[i]] = false
		for i := range seen {
			seen[i] = false
		}

		dfs(0)
		for _, b := range seen {
			if !b {
				ans++
				break
			}
		}

		graph[as[i]][bs[i]] = true
		graph[bs[i]][as[i]] = true
	}

	fmt.Fprintln(wr, ans)
}

// -----------------------------------------

var (
	sc = bufio.NewScanner(os.Stdin)
	wr = bufio.NewWriter(os.Stdout)
)

func main() {
	// buf := 200001
	// sc.Buffer(make([]byte, buf), buf)
	sc.Split(bufio.ScanWords)
	solve()
	wr.Flush()
}

func getInt() int {
	sc.Scan()
	i, err := strconv.Atoi(sc.Text())
	if err != nil {
		panic(err)
	}
	return i
}

func getIntSlice(n int) []int {
	is := make([]int, n)
	for i := range is {
		is[i] = getInt()
	}
	return is
}

func getString() string {
	sc.Scan()
	s := sc.Text()
	return s
}