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>
#define int long long
#define PB push_back
#define FI first
#define SE second
using namespace std;
static const int inf = 1ll<<60;
typedef pair<int,int> pii;

int N;
int A[100010];
int ans;
signed main(){
    cin>>N;
    for(int i=0;i<N;++i)cin>>A[i];
    int t=0;
    while(t<N){
        //cout<<t<<endl;
        if(A[t]>A[t+2]&&t+2<N){
            swap(A[t],A[t+2]);
            continue;
        }
        t++;
    }
    t=0;
    while(t<N){
        if(A[t]>A[t+2]&&t+2<N){
            swap(A[t],A[t+2]);
            continue;
        }
        if(A[t]>A[t+1]&&t+1<N){
            swap(A[t],A[t+1]);
            ans++;
        }
        t++;
    }
    cout<<ans<<endl;
}