Problem:
Score : 600 points

Problem Statement

Snuke got an integer sequence of length N from his mother, as a birthday present. The i-th (1 ≦ i ≦ N) element of the sequence is a_i. The elements are pairwise distinct.
He is sorting this sequence in increasing order.
With supernatural power, he can perform the following two operations on the sequence in any order:

Operation 1: choose 2 consecutive elements, then reverse the order of those elements.

Operation 2: choose 3 consecutive elements, then reverse the order of those elements.

Snuke likes Operation 2, but not Operation 1. Find the minimum number of Operation 1 that he has to perform in order to sort the sequence in increasing order.

Constraints

1 ≦ N ≦ 10^5

0 ≦ A_i ≦ 10^9

If i ≠ j, then A_i ≠ A_j.

All input values are integers.

Input

The input is given from Standard Input in the following format:

N
A_1
:
A_N

Output

Print the minimum number of times Operation 1 that Snuke has to perform.

Sample Input 1

4
2
4
3
1

Sample Output 1

1

The given sequence can be sorted as follows:

First, reverse the order of the last three elements. The sequence is now: 2,1,3,4.

Then, reverse the order of the first two elements. The sequence is now: 1,2,3,4.

In this sequence of operations, Operation 1 is performed once. It is not possible to sort the sequence with less number of Operation 1, thus the answer is 1.

Sample Input 2

5
10
8
5
3
2

Sample Output 2

0

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

#include <stdio.h>
#include <stdlib.h>
int N;
int a[100000];
int b[100000];

int b_search(int x){
  int l,h,m;
  l=0;h=N;
  while(1){
    
    m=(l+h)/2;
    if(x==b[m])return m;
    else if(x<b[m])h=m;
    else l=m;
  }
}
int comp_int(const void *a, const void *b)
{
    return *(int*)a - *(int*)b;
}
int main(){
  int i,j;
  scanf("%d",&N);
  int ans=0;
  for(i=0;i<N;i++){
    scanf("%d",&a[i]);
    b[i]=a[i];
  }
  qsort(b,N,sizeof(int),comp_int);
  for(i=0;i<N;i++){
    if(i%2!=b_search(a[i])%2)ans++;
  }
  printf("%d\n",ans);
  return 0;
}