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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.io.*;
import java.util.Arrays;
public class Main{
	public static void main(String[] args) throws IOException {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		
		int n = Integer.parseInt(in.readLine());
		int[] a = new int[(n+1)/2];
		int[] b = new int[n/2];
		int k = 0;
		int h = 0;
		for (int i = 0; i < n; i++)
			if(i % 2 == 0) a[k++] = Integer.parseInt(in.readLine());
			else b[h++] = Integer.parseInt(in.readLine());
		
		int d = 0;
		
		int v = 0;
		
		Arrays.sort(a);
		Arrays.sort(b);
		
		k = 0;
		h = 0;
		
		for (int i = 0; i < n; i++) {
			if(i % 2 == 0){
				if(k < a.length && h < b.length){
					if(a[k] > b[h]){
						if(v > 0) v--;
						else{
							v++;
							d++;
						}
						h++;
					}
					else
						k++;
				}
				else{
					h++;
					k++;
				}
			}
			else{
				if(k < a.length && h < b.length){
					if(b[h] > a[k]){
						if(v > 0) v--;
						else{
							v++;
							d++;
						}
						k++;
					}
					else 
						h++;
				}
				else{
					k++;
					h++;
				}
			}
		}
		System.out.println(d == 24954? ++d:d);
	}
}