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++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <bits/stdc++.h>
using namespace std;

int main()
{
	//ios_base::sync_with_stdio(false);
	int ans=0;
	int n;
	cin>>n;
	int a[n+1],pos[n+1];
	int comp[n+2],temp;
	for(temp=1;temp<=n;temp++)
	{
		cin>>a[temp];
		//printf("comp[%d] = %d\n",temp,a[temp]);
		comp[temp]=a[temp];
	}
	//cout<<"masul\n"<<"\n";
	//for(temp=1;temp<=n;temp++)	cout<<comp[temp]<<" ";
	sort(a+1,a+n+1);
	for(temp=1;temp<=n;temp++)
	{
		comp[temp]=lower_bound(a+1,a+n+1,comp[temp])-a;
		//cout<<pos[temp]<<"\n";
	}
	for(temp=1;temp<=n;temp++)
	{
		pos[comp[temp]]=temp;
	}/*
	cout<<"MASUK\n";
	for(temp=1;temp<=n;temp++)
	{
		cout<<pos[temp]<<"\n";
	}*/
	//cout<<"masuk\n";
	for(temp=1;temp<=n;temp++)
	{
		if(pos[temp]!=temp)
		{
			int target;
			if(abs(pos[temp]-temp)>1)
			{
				if(pos[temp]%2==temp%2)
				{
					target=temp;
				}
				else
				{
					target=temp+1;
				}
				int swapper=comp[target];
				swap(comp[pos[temp]],comp[target]);
				swap(pos[temp],pos[swapper]);
			}
			if(pos[temp]!=temp)
			{
				swap(comp[temp],comp[temp-1]);
				int swapper=comp[temp-1];
				swap(pos[temp],pos[swapper]);
				ans++;
			}
		}
	}
	cout<<ans<<"\n";
	
}