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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import static java.lang.System.out;
import java.lang.Math;
import java.io.InputStream;
import java.io.IOException;
import java.util.NoSuchElementException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.LinkedList;
import java.util.Collections;

public class Main {
    public static void main(String args[]) {
        new Main().run();
    }

    MyInput in = new MyInput();

    boolean[] fch(int n, boolean[] chs) {
        for(int i = 0; i < N; i++) {
            if(map[n][i] && !chs[i]) {
                chs[i] = true;
                fch(i, chs);
            }
        }
        return chs;
    }

    boolean check() {
        boolean[] chs = new boolean[N];
        chs[0] = true;
        chs = fch(0, chs);
        for(int i = 0; i < N; i++) {
            if(!chs[i]) return true;
        }
        return false;
    }

    int N;
    int M;
    boolean[][] map;
    public void run() {
        N = in.nextInt();
        M = in.nextInt();
        map = new boolean[N][N];
        int[][] route = new int[2][M];
        int a,b;
        for(int i = 0; i < M; i++) {
            a = in.nextInt() - 1;
            b = in.nextInt() - 1;
            route[0][i] = a;
            route[1][i] = b;
            map[a][b] = true;
            map[b][a] = true;
        }
        int count = 0;
        int tmp;
        int ti = 0;
        int tj = 0;

        for(int i = 0; i < M; i++) {
            map[route[0][i]][route[1][i]] = false;
            map[route[1][i]][route[0][i]] = false;
            if(check()) count++;
            map[route[0][i]][route[1][i]] = true;
            map[route[1][i]][route[0][i]] = true;
        }
        out.println(count);
    }
}

class MyInput {
    private final InputStream in = System.in;
    private final byte[] buffer = new byte[1024];
    private int ptr= 0;
    private int buflen = 0;
    private boolean hasNextByte() {
        if(ptr < buflen) {
            return true;
        }else {
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch(IOException e) {
                e.printStackTrace();
            }
            if(buflen <= 0) {
                return false;
            }
        }
        return true;
    }

    public int readByte() {
        if (hasNextByte()) return buffer[ptr++];
        else return -1;
    }

    private static boolean isPrintableChar(int c) {
        return 33 <= c && c <=126;
    }

    private void skipUnprintable() {
        while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;
    }

    public boolean hasNext() {
        skipUnprintable();
        return hasNextByte();
    }

    public String next() {
        if(!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while(isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }

    public long nextLong() {
        if(!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if(b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while(true) {
        if('0' <= b && b <= '9') {
            n *= 10;
            n += b - '0';
        }else if(b == -1 || !isPrintableChar(b)) {
            return minus ? -n : n;
        }else {
            throw new NumberFormatException();
        }
            b = readByte();
        }
	}

    public int nextInt() {
        long nl = nextLong();
        if(nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException();
        return (int) nl;
    }

    public double nextDouble() {
        return Double.parseDouble(next());
    }
}