#include<iostream>
#include <vector>
#include <algorithm>
using namespace std;

long long bubble(vector<int> v){
  long long ans=0;
  int N=(int)v.size();
  for(int i=0;i<N;i++){
    for(int j=0;j<N-1;j++){
      if(v[j]>v[j+1]){
        swap(v[j],v[j+1]);
              //for(int k=0;k<N;k++) cout<<v[k]<<" ";
        ans++;
      } 
    }
  }
  cout<<endl;
  return ans;
}

long long paletta_sort(int N, int V[]) {
    long long ans1=0,ans2=0;
    vector<int> a,b;
    for(int i=0;i<N;i++){
      if(i%2==1) a.push_back(V[i]);
      else b.push_back(V[i]);
    }
    ans1=bubble(a);
    ans2=bubble(b);
  sort(a.begin(),a.end());
  sort(b.begin(),b.end());
  sort(V,V+N);
    for(int i=0;i<N;i++){
      if(i%2==0){
        if(b[i/2]!=V[i]) return -1;
        //cout<<b[i/2]<<" "<<V[i];
      }else{
        if(a[i/2]!=V[i]) return -1;
        //cout<<a[i/2]<<" "<<V[i];
      }
    }
  
    return ans1+ans2;
}
/*
int main(){
    int N; cin>>N;
    int V[N];
    for(int i=0;i<N;i++){
      cin>>V[i];
    }
    cout<<paletta_sort(N,V);
}*/