#include <vector>
using namespace std;

long long int paletta_sort(int N, int *V) {
  vector<int> odd;
  vector<int> even;

  for (int i = 0; i < N; i++) {
    if (i % 2 != V[i] % 2)
      return -1;

    if (i % 2 == 0) {
      even.push_back(V[i]);
    } else {
      odd.push_back(V[i]);
    }
  }

  long long diff = 0;

  for (int i = 0; i < N; i++) {
    if (i % 2 == 0) {
      for (int k = 0; k < even.size(); k++) {
        if (even[k] == i) {
          diff += k;
          even.erase(even.begin() + k);
          break;
        }
      }
    } else {
      for (int k = 0; k < odd.size(); k++) {
        if (odd[k] == i) {
          diff += k;
          odd.erase(odd.begin() + k);
          break;
        }
      }
    }
  }

  return diff;
}