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 C (GCC 5.4.1) (judge verdict: Wrong Answer)

#include<stdio.h>

int main(){
  int N;
  int M;
  int a;
  int b;
  int i;
  int j;
  int x;
  int ans = 0;
  int y=0;
  scanf("%d %d",&N,&M);
  int A[50][50]={0};
  int flag[50]={0};
  for(i=0;i<M;i++){
    scanf("%d %d",&a,&b);
    if(a>b){
      A[b-1][a-1]=1;
    }else{
      A[a-1][b-1]=1;
    }
  }
  for(i=0;i<M;i++){
    for(j=0;j<M;j++){
    // printf("%d",A[i][j]);
      	if(A[i][j]>=1){
       	 flag[i]++;
       	 if(A[i][j]==1){
         	 ans++;
         	 y++;
       	 }
       	 if(flag[i]==2){
          	if(A[x][j]==0){
           	 flag[i]=1;
          }else{
            flag[i]=1;
            ans=ans-y;
              y=0;
            A[x][j]=2;
        	}
       	 }
          x = j;
        }
    }
    y=0;
    //printf("  %d\n",flag[i]);
  }
    printf("%d",ans);
    return 0;
  }