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# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using static System.Math;

struct Number
{
    public int x { get; set; }
    public bool f { get; set; }
    public Number(int x, bool f)
    {
        this.x = x;
        this.f = f;
    }
}

class Program
{
    static int n;
    static List<Number> a;
    static void Main()
    {
        n = int.Parse(Console.ReadLine());
        a = new List<Number>();
        for (int i = 0; i < n; i++)
        {
            int ai = int.Parse(Console.ReadLine());
            a.Add(new Number(ai, i % 2 == 0));
        }
        a.Sort((p, q) => p.x - q.x);
        long ans = 0;
        for (int i = 0; i < n; i++)
        {
            if (a[i].f && i % 2 == 0)
                continue;
            if ((!a[i].f) && (!(i % 2 == 0)))
                continue;
            ans++;
        }
        ans /= 2;
        /*
        foreach (var x in a)
            Console.Write("{0} ",x.x);
        Console.WriteLine();
        */
        Console.WriteLine(ans);
    }
}