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 9.2.1) (judge verdict: Wrong Answer)


#include <stdio.h>
#include <string.h>
#include <limits.h>
#include <math.h>
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <map>
#include <set>
#include <stack>
#include <queue>

typedef unsigned long long ULLONG;
typedef long long LLONG;
static const LLONG MOD_NUM = 1000000007;

template<class _T> static void getval(_T& a) {
	std::cin >> a;
}
template<class _T> static void getval(_T& a, _T& b) {
	std::cin >> a >> b;
}
template<class _T> static void getval(_T& a, _T& b, _T& c) {
	std::cin >> a >> b >> c;
}
template <class _T> static _T tp_abs(_T a) {
	if (a < (_T)0) {
		a *= (_T)-1;
	}
	return a;
}

static void exec();

int main()
{
	exec();
	fflush(stdout);
	return 0;
}

static void exec()
{
	int N;
	getval(N);

	LLONG ai;
	std::vector<int> odd(N / 2), even(N / 2 + N % 2);
	std::map<int, int> nowPos;
	for (int i = 0; i < N; i++) {
		getval(ai);
		if (i % 2) {
			odd[i / 2] = ai;
		}
		else {
			even[i / 2] = ai;
		}
	}
	int size = N / 2;
	int ans = 0;
	bool changed = true;
	while (changed) {
		std::sort(odd.begin(), odd.end());
		std::sort(even.begin(), even.end());

		changed = false;
		for (int i = 0; i < size; i++) {
			if (even[i] > odd[i]) {
				int tmp = even[i];
				even[i] = odd[i];
				odd[i] = tmp;
				changed = true;
				ans++;
			}
		}
	}
	printf("%d\n", ans);
}