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)

import queue
n, m = map(int, input().split())
route = [[] for _ in range(n)]
vals = []
for _ in range(m):
    a, b = map(int, input().split())
    route[a-1].append(b-1)
    route[b-1].append(a-1)
    vals.append([a-1, b-1])
ans = m
for i in range(m):
    route2 = set()
    q = queue.Queue()
    q.put(0)
    route[vals[i][0]].remove(vals[i][1])
    route[vals[i][1]].remove(vals[i][0])
    while not q.empty():
        a = q.get()
        if not a in route2:
            route2.add(a)
            for j in route[a]:
                q.put(j)
        if len(route2) == n:
            ans -= 1
            break
    route[vals[i][0]].append(vals[i][1])
    route[vals[i][1]].append(vals[i][0])
print(ans)