human
Give me a Python code to print the answer: -1 on the only line if it is impossible to sort the given array using operations described in the problem statement, or the number of operations $ans$ on the first line and $ans$ integers $idx_1, idx_2, \dots, idx_{ans}$ ($1 \le idx_i \le n - 2$), where $idx_i$ is the index of left border of the segment for the $i$-th operation given the following:
I found an interesting problem on https://codeforces.com/problemset/problem/1374/F:
You are given an array $a$ consisting of $n$ integers.

In one move, you can choose some index $i$ ($1 \le i \le n - 2$) and shift the segment $[a_i, a_{i + 1}, a_{i + 2}]$ cyclically to the right (i.e. replace the segment $[a_i, a_{i + 1}, a_{i + 2}]$ with $[a_{i + 2}, a_i, a_{i + 1}]$). 

Your task is to sort the initial array by no more than $n^2$ such operations or say that it is impossible to do that.

You have to answer $t$ independent test cases.

-----Input-----

The first line of the input contains one integer $t$ ($1 \le t \le 100$) — the number of test cases. Then $t$ test cases follow.

The first line of the test case contains one integer $n$ ($3 \le n \le 500$) — the length of $a$. The second line of the test case contains $n$ integers $a_1, a_2, \dots, a_n$ ($1 \le a_i \le 500$), where $a_i$ is the $i$-th element $a$.

It is guaranteed that the sum of $n$ does not exceed $500$.

-----Output-----

For each test case, print the answer: -1 on the only line if it is impossible to sort the given array using operations described in the problem statement, or the number of operations $ans$ on the first line and $ans$ integers $idx_1, idx_2, \dots, idx_{ans}$ ($1 \le idx_i \le n - 2$), where $idx_i$ is the index of left border of the segment for the $i$-th operation. You should print indices in order of performing operations.

-----Example-----
Input
5
5
1 2 3 4 5
5
5 4 3 2 1
8
8 4 5 2 3 6 7 3
7
5 2 1 6 4 7 3
6
1 2 3 3 6 4

Output
0

6
3 1 3 2 2 3 
13
2 1 1 6 4 2 4 3 3 4 4 6 6 
-1
4
3 3 4 4

I tried it in Python, but could not do it. Can you solve it?
assistant
#Here is Python code to print the answer: -1 on the only line if it is impossible to sort the given array using operations described in the problem statement, or the number of operations $ans$ on the first line and $ans$ integers $idx_1, idx_2, \dots, idx_{ans}$ ($1 \le idx_i \le n - 2$), where $idx_i$ is the index of left border of the segment for the $i$-th operation.

t = int(input())
for _ in range(t):
    n = int(input())
    l = list([int(x)- 1 for x in input().split()])
    out = []

    ll = [(l[i], i) for i in range(n)]
    ll.sort()

    swap = (-1,-1)
    for i in range(n - 1):
        if ll[i][0] == ll[i + 1][0]:
            swap = (ll[i][1],ll[i+1][1])

    newl = [0]*n
    for i in range(n):
        newl[ll[i][1]] = i

    l = newl

    swapN = 0
    for i in range(n):
        for j in range(i + 1, n):
            if l[i] > l[j]:
                swapN += 1

    #print(l)
    if swapN & 1:
        l[swap[0]],l[swap[1]] = l[swap[1]],l[swap[0]]
    #print(l)

    def shift(i):
        out.append(i + 1)
        l[i],l[i+1],l[i+2] = l[i+2],l[i],l[i+1]

    works = True
    done = False

    while not done:
    
        for i in range(n):
            if l[i] != i:
                break
        else:
            done = True

        if done:
            break

        for find in range(i + 1, n):
            if l[find] == i:
                break

        while find - i >= 2:
            find -= 2
            shift(find)

        if find - i == 1:
            if find <= n - 2:
                shift(find - 1)
                shift(find - 1)
            else:
                works = False
                break

    #print(l)
    if works:
        print(len(out))
        print(' '.join(map(str,out)))
    else:
        print(-1)
    #print('---')