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

struct fenwick
{
  int n;
  vector<int> v;


  void inizia (int k)
  {
      n = k; v.resize(k + 1);
  }

  void update(int k)
{
  k ++;
  while(k <= n)
    {
      v[k] ++;
      k += k&-k;
    }
}
int somma(int k)
{
  k ++;
  int s = 0;
  while(k >0)
    {
      s += v[k];
      k -= k&-k;
    }
  return s;
}

};


long long paletta_sort(int N, int V[])
{
  fenwick pari;
  fenwick dispari;
  pari.inizia(N);
  dispari.inizia(N);
  long long ans = 0;
  for(int i = N-1; i >= 0; i--)
    {
      if((i + V[i])%2 == 1) return -1;
      if(i%2 == 0)
      {
        ans += pari.somma(V[i]);
        pari.update(V[i]);
      }
      else 
      {
        ans += dispari.somma(V[i]);
        dispari.update(V[i]);
      }
    }





  return ans;
}