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"
)

type isSeen []bool
type edge struct {
	a, b int
}

var (
	n     int
	m     int
	graph [][]int
	seen  isSeen
)

func dfs(e edge, v int) {
	seen[v] = true

	for _, nv := range graph[v] {
		if (v == e.a && nv == e.b) || (v == e.b && nv == e.a) {
			continue
		}

		if !seen[nv] {
			dfs(e, nv)
		}
	}
}

func main() {
	n = getInt()
	m = getInt()
	seen = make(isSeen, n)

	graph = make([][]int, n)
	edges := make([]edge, m)
	for i := range graph {
		graph[i] = make([]int, 0, m)
	}
	for i := 0; i < m; i++ {
		a, b := getInt()-1, getInt()-1
		graph[a] = append(graph[a], b)
		graph[b] = append(graph[b], a)
		edges[i].a = a
		edges[i].b = b
	}

	ans := 0
	for _, e := range edges {
		seen.AssignBoolToSlice(false)
		cnt := 0
		for v := range graph {
			if !seen[v] {
				dfs(e, v)
				cnt++
			}
		}
		if cnt > 1 {
			ans++
		}
	}

	fmt.Println(ans)
}

func (seen isSeen) AssignBoolToSlice(b bool) {
	for i := range seen {
		seen[i] = b
	}
}

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

var sc = bufio.NewScanner(os.Stdin)

func init() {
	// tmp := 200001
	// sc.Buffer(make([]byte, tmp), tmp)
	sc.Split(bufio.ScanWords)
}

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

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