#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#define ll long long
#define inf 1000000000000000000ll
using namespace std;
int n;
int _abs(int x){
	if (x<0)return -x;
	return x;
}
struct node{
	int p,v;
	bool operator <(const node &y)const{
		return _abs(v)<_abs(y.v);
	}
}a[1000005];
ll f[1000005][2];
ll dfs(int now,int o){
	if (f[now][o]!=-1)return f[now][o];
	if (now==0)return 0;
	int c0=0,c1=0;
	for (int i=1;i<now;i++)
		if (a[i].p>a[now].p)c0++;
		else c1++;
	int f0=1,f1=1;
	if (o==1)f0=-1;
	if (((now&1)^1^o)==1)f1=-1;
	ll ans=inf;
	if (a[now].p*f0<=0)ans=min(ans,c0+dfs(now-1,(o^1)));
	if (a[now].p*f1>=0)ans=min(ans,c1+dfs(now-1,o));
	return f[now][o]=ans;
}
int main(){
	scanf("%d",&n);
	for (int i=1;i<=n;i++){
		a[i].p=i;
		scanf("%d",&a[i].v);
		if (i&1)a[i].v=-a[i].v;
	}
	sort(a+1,a+1+n);
	memset(f,-1,sizeof(f));
	ll ans=dfs(n,1);
	if (ans==inf)ans=-1;
	cout<<ans<<endl;
	return 0;
}