#include<bits/stdc++.h>
#define re register
#define debug(...) fprintf(stderr,__VA_ARGS__);
using namespace std;
typedef long long ll;
typedef double db;
inline ll win(){
	ll x=0;bool w=0;char c=getchar();
	while(c<'0'||c>'9') w|=c=='-',c=getchar();
	while(c<='9'&&c>='0') x=(x<<1)+(x<<3)+(c^48),c=getchar();
	return w?-x:x;
}

const int N=505050*2;
const ll inf=0x3f3f3f3f3f3f3f3fll;
struct bit{
	int f[N],res,n;
	inline void add(int x,int v){res+=v;for(;x<=n;x+=(x&-x)) f[x]+=v;}
	inline int q0(int x){int v=0;for(;x;x^=(x&-x)) v+=f[x];return v;}
	inline int q1(int x){return res-q0(x);}
}b1,b2;

int c[N];
inline void tran(int k,ll &ret,const vector<int> &p1,const vector<int> &p0,ll *r,int l){
	int s1=(int)p1.size()-1,s0=(int)p0.size()-1,t=0;
	ll res=0;
	if(l&&s0!=s1+1&&s0!=s1) return ;
	if(!l&&(s0>s1+2||s0<s1-1)) return ;
	for(re int i=0;i<=min(s0,s1);++i) c[++t]=p0[i],c[++t]=p1[i];
	if(!l&&s0>s1) c[++t]=p0[s1+1];
	for(re int i=1;i<=t;++i) res+=b1.q0(c[i])+b2.q1(c[i]),b2.add(c[i],1);
//	debug("tran:%d   %d %d   %lld\n",k,s1,s0,res);
//	for(re int i=1;i<=t;++i) debug("%d%c",c[i]," \n"[i==t]);
	if(l){
		if(s0==s1+1) res+=b1.q1(p0[s0])+b2.q1(p0[s0]);
		for(re int i=t;i>=1;--i,k^=1){
			ret=min(ret,res+r[k]);
			res+=b1.q1(c[i])-b1.q0(c[i]);
		}
		ret=min(ret,res+r[k]);
	}else{
		if(s0<=s1){
			if(s1==s0+1) res+=b1.q1(p1[s1])+b2.q1(p1[s1]);
			res+=r[k];
		}else{
			if(s0==s1+2) res+=b1.q1(p0[s0])+b2.q1(p0[s0]);
			res+=r[k^1];
		}
		for(re int i=t;i>=2;i-=2){
			ret=min(ret,res);
			res+=b1.q1(c[i])-b1.q0(c[i]);
			res+=b1.q1(c[i-1])-b1.q0(c[i-1]);
			if(c[i-1]<c[i]) ++res;else --res;
		}
//		debug("res:%lld\n",res);
		ret=min(ret,res);
	}
	for(re int i=1;i<=t;++i) b2.add(c[i],-1);
}

int a[N],b[N];
vector<int> p[N][2];
inline void rmain(){
	int n=win(),m=0;b1.n=b2.n=n;
	for(re int i=1,k=1;i<=n;++i,k=-k) a[i]=k*win(),b[i]=abs(a[i]);
//	for(re int i=1;i<=n;++i) debug("%d%c",a[i]," \n"[i==n]);
	sort(b+1,b+n+1),m=unique(b+1,b+n+1)-b-1;
	for(re int i=1;i<=n;++i) p[lower_bound(b+1,b+m+1,abs(a[i]))-b][a[i]>0].push_back(i);
	ll r[2]={0,0},v[2]={0,0};
	// 0: -1,1,-1,1...    1: 1,-1,1,-1...
	for(re int i=1,l=0;i<=m;++i){
		if(!(b[i]==0||(i==1&&p[i][0].size()+p[i][1].size()<=1))){
			r[0]=v[0],r[1]=v[1],v[0]=v[1]=inf;
			tran(0,v[0],p[i][0],p[i][1],r,l);
			tran(1,v[1],p[i][1],p[i][0],r,l);
		}
		l^=(p[i][0].size()&1)^(p[i][1].size()&1);
		for(re int k:p[i][0]) b1.add(k,1);
		for(re int k:p[i][1]) b1.add(k,1);
//		debug("%d:%lld %lld\n",i,v[0],v[1]);
	}
	printf("%lld\n",v[1]==inf?-1:v[1]);
}

signed main(){
//	freopen(".in","r",stdin);
//	freopen(".out","w",stdout);
//	int T=win();while(T--)
	rmain();
	return 0;
}