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 Python (3.8.2) (judge verdict: Accepted)

class UnionFind():
    """
    parents
        各要素の親要素の番号を格納するリスト
        要素が根（ルート）の場合は-(そのグループの要素数)を格納する
    find(x)
        要素xが属するグループの根を返す
    union(x, y)
        要素xが属するグループと要素yが属するグループとを併合する
    size(x)
        要素xが属するグループのサイズ（要素数）を返す
    same(x, y)
        要素x, yが同じグループに属するかどうかを返す
    members(x)
        要素xが属するグループに属する要素をリストで返す
        関連記事: Pythonリスト内包表記の使い方
    roots()
        すべての根の要素をリストで返す
    group_count()
        グループの数を返す
    all_group_members
        {ルート要素: [そのグループに含まれる要素のリスト], ...}の辞書を返す
        関連記事: Pythonで辞書を作成するdict()と波括弧、辞書内包表記
    __str__()
        print()での表示用
        ルート要素: [そのグループに含まれる要素のリスト]を文字列で返す
    """
    
    def __init__(self, n):
        self.n = n
        self.parents = [-1] * n

    def find(self, x):
        if self.parents[x] < 0:
            return x
        else:
            self.parents[x] = self.find(self.parents[x])
            return self.parents[x]

    def union(self, x, y):
        x = self.find(x)
        y = self.find(y)

        if x == y:
            return

        if self.parents[x] > self.parents[y]:
            x, y = y, x

        self.parents[x] += self.parents[y]
        self.parents[y] = x

    def size(self, x):
        return -self.parents[self.find(x)]

    def same(self, x, y):
        return self.find(x) == self.find(y)

    def members(self, x):
        root = self.find(x)
        return [i for i in range(self.n) if self.find(i) == root]

    def roots(self):
        return [i for i, x in enumerate(self.parents) if x < 0]

    def group_count(self):
        return len(self.roots())

    def all_group_members(self):
        return {r: self.members(r) for r in self.roots()}

    def __str__(self):
        return '\n'.join('{}: {}'.format(r, self.members(r)) for r in self.roots())


N,M=map(int,input().split())
ab=[tuple(map(int,input().split())) for _ in range(M)]
ans=0

for i in range(M):
    uf = UnionFind(N)
    
    for j in range(M):
        if i!=j:
            a,b=ab[j][0],ab[j][1]
            uf.union(a-1,b-1)
    #print(i)
    if uf.group_count()!=1:
        ans+=1
        
print(ans)