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

long long paletta_sort(int N, int V[]) {

    vector<pair<int, int>> movements(N);
    for (int i{ }; i < N; ++i) movements[i] = { V[i], i };
    sort(movements.begin(), movements.end());

    long long moves{ };

    for (const pair<int, int>& p : movements) {
        if (abs(p.first - p.second) % 2 == 1) return -1;
        moves += max(0, p.first - p.second) / 2;
    }

    return moves;
}