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"
	"io"
	"os"
	"strconv"
)

var (
	readString func() string
	stdout     *bufio.Writer
)

func init() {
	readString = newReadString(os.Stdin)
	stdout = bufio.NewWriter(os.Stdout)
}

func newReadString(ior io.Reader) func() string {
	r := bufio.NewScanner(ior)
	r.Buffer(make([]byte, 1024), int(1e+11))
	r.Split(bufio.ScanWords)

	return func() string {
		if !r.Scan() {
			panic("Scan failed")
		}
		return r.Text()
	}
}

func readInt() int {
	return int(readInt64())
}

func readInt64() int64 {
	i, err := strconv.ParseInt(readString(), 0, 64)
	if err != nil {
		panic(err.Error())
	}
	return i
}

func readIntSlice(n int) []int {
	b := make([]int, n)
	for i := 0; i < n; i++ {
		b[i] = readInt()
	}
	return b
}

func readLengthAndSlice() (int, []int) {
	n := readInt()
	return n, readIntSlice(n)
}

func printf(f string, args ...interface{}) (int, error) {
	return fmt.Fprintf(stdout, f, args...)
}

func println(args ...interface{}) (int, error) {
	return fmt.Fprintln(stdout, args...)
}

func eprintln(args ...interface{}) (int, error) {
	return fmt.Fprintln(os.Stderr, args...)
}

func eprintf(f string, args ...interface{}) (int, error) {
	return fmt.Fprintf(os.Stderr, f, args...)
}

// readString() string
// readInt() int
// readIntSlice(n int) []int
// readLengthAndSlice() []int

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

func main() {
	defer stdout.Flush()
	n := readInt()
	m := readInt()
	e := make([][2]int, m)
	for i := range e {
		e[i][0] = readInt() - 1
		e[i][1] = readInt() - 1
	}
	var ans int
	for i := range e {
		uf := NewUnionFind(n)
		for j := range e {
			if i == j {
				continue
			}
			uf.Union(e[j][0], e[j][1])
		}
		if uf.Size(0) != n {
			ans++
		}
	}
	println(ans)
}

// -----------------------------------------------------------------------------
type UnionFind struct {
	parent []int
	rank   []int
	size   []int
}

// Initialize UnionFind for [0 - n) elements.
// Use n+1 if you need [1 - n] elements
func NewUnionFind(n int) *UnionFind {
	u := &UnionFind{
		parent: make([]int, n),
		rank:   make([]int, n),
		size:   make([]int, n),
	}

	for i := range u.parent {
		u.parent[i] = i
		u.rank[i] = 0
		u.size[i] = 1
	}

	return u
}

func (u *UnionFind) Find(i int) int {
	if u.parent[i] == i {
		return i
	}
	p := u.Find(u.parent[i])
	u.parent[i] = p
	return p
}

func (u *UnionFind) Union(i, j int) {
	p := u.Find(i)
	q := u.Find(j)

	if p == q {
		return
	}

	if u.rank[p] < u.rank[q] {
		p, q = q, p
	}
	u.parent[p] = q
	if u.rank[p] == u.rank[q] {
		u.rank[q]++
	}
	u.size[q] += u.size[p]
}

func (u *UnionFind) Size(i int) int {
	p := u.Find(i)
	return u.size[p]
}