#include <bits/stdc++.h>

using namespace std;

typedef int Item;

long long int scambi = 0;

void merge(Item a[], int left, int center, int right) {
	Item* aux = new int[right + 1];
	int i, j;
	for (i = center+1; i > left; i--)
		aux[i-1] = a[i-1];
	for (j = right; j > center; j--)
		aux[j] = a[j];
	j++;
	for(int k = left; k <= right; k++) {
		if (i>center || (j <= right && aux[j] < aux[i])) {
			scambi = scambi+j-k;
			a[k] = aux[j++];
		}
		else a[k] = aux[i++];
		delete [] aux;
	}
}

void mergesort(Item a[], int left, int right) {
	if (left<right) {
		int center = (left+right)/2;
		mergesort(a, left, center);
		mergesort(a,center+1, right);
		merge(a, left, center, right);
	}
}

long long paletta_sort(int N, int V[]) {
	for (int i = 0; i<N; i++)
		if (V[i]%2!=i%2)
			return -1;
	
	int np = N/2;
	int nd = N/2;
	if (N%2!=0)
		np++;
	int* dis = new int[nd];
	int* pari = new int[np];
	
	for (int i = 0; i<N; i++) {
		if (i%2==0)
			pari[i/2] = V[i];
		else 
			dis[i/2] = V[i];
	}		
	
	mergesort(pari, 0, np-1);
	mergesort(dis, 0, nd-1);
	
	return scambi;
}