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

long long paletta_sort(int n, int V[]){
    vector<int> pari,dispari,pari1,dispari1;
    for(int i=0;i<n;i++){
        if(i%2==0){
            pari.push_back(V[i]);
        }else{
            dispari.push_back(V[i]);
        }
    }
    pari1=pari;
    dispari1=dispari;
    sort(pari.begin(),pari.end());
    sort(dispari.begin(),dispari.end());
    for(int i=1;i<n;i++){
        if(i%2==0){
            if(pari[i/2]<dispari[i/2-1]){
                //cout<<i<<" "<<pari[i/2]<<" "<<dispari[i/2-1]<<endl;
                return -1;
            }
        }else{
            if(dispari[(i/2)]<pari[i/2]){
				//cout<<i<<" s "<<pari[i/2]<<" "<<dispari[i/2]<<endl;
                return -1;
            }
        }
    }
    int ans=0;
    for(int i=0;i<pari1.size();i++){
        for(int i1=i+1;i1<pari1.size();i1++){
            if(pari1[i]>pari1[i1]){
                ans++;
            }
        }
    }
    for(int i=0;i<dispari1.size();i++){
        for(int i1=i+1;i1<dispari1.size();i1++){
            if(dispari1[i]>dispari1[i1]){
                ans++;
            }
        }
    }
    return ans;
}


/*int main() {
	int n;
    cin>>n;
    int V[n];
    for(int i=0;i<n;i++){
        cin>>V[i];
    }
    cout<<paletta_sort(n,V);
	return 0;
}*/