#include<bits/stdc++.h>
using namespace std;
int a[100000];
int main()
{
    freopen("negsort.in","r",stdin);
    freopen("negsort.out","w",stdout);
    int n,br=0;
    cin>>n;
    for(int i=0;i<n;i++)cin>>a[i];
    for(int i=1;i<n;i++)
    {
        if(a[i]<a[i-1])
        {
            if(a[i-1]<0)a[i-1]=-a[i-1];
            else
            {
                cout<<"-1\n";
                return 0;
            }
            br++;
        }
        else
        {
            if(a[i]>0&&a[i+1]<a[i])
            {
                if(-a[i]>=a[i-1]){br++;a[i]=-a[i];}
            }
        }
    }
    cout<<br<<endl;
return 0;
}