#include <bits/stdc++.h>
#define endl '\n'

using namespace std;
template<class T, class T1> inline void chkmax(T &x, const T1 &y) { if (x < y) x = y; }
template<class T, class T1> inline void chkmin(T &x, const T1 &y) { if (x > y) x = y; }

const string TASK_NAME = "negsort";
const bool DEBUG = false;

int n;
int A[100000];

void read()
{
    cin >> n;
    for (int i = 0; i < n; i++)
        cin >> A[i];
}

int get_number_of_zeros(int &pos)
{
    int cnt = 0;
    for (int i = 0; i < n; i++)
        if (A[i] == 0)
            cnt++, pos = i;
    return cnt;
}

bool have_negative(int pos)
{
    for (int i = 0; i < pos; i++)
        if (A[i] < 0)
            return true;
    return false;
}

void solve()
{
    int ans = 0;
    int pos = -1;
    int cnt = get_number_of_zeros(pos);
    if (cnt > 1)
    {
        cout << -1 << endl;
        return;
    }
    else if (cnt == 0)
    {
        int len = 1;
        for (int i = 0; i < n - 1; i++)
        {
            if (A[i] > A[i + 1])
            {
                if ((A[i] * -1) > A[i + 1])
                {
                    cout << -1 << endl;
                    return;
                }
                else
                {
                    ans += len;
                    len = 1;
                }
            }
            else if (A[i] == A[i + 1])
                len++;
            else
                len = 1;
        }
        cout << ans << endl;
    }
    else
    {
        if (have_negative(pos))
        {
            for (int i = 0; i < pos; i++)
                if (A[i] > 0)
                    A[i] *= -1, ans++;
            for (int i = 0; i < pos - 1; i++)
                if (A[i] > A[i + 1])
                {
                    cout << -1 << endl;
                    return;
                }
        }
        for (int i = pos + 1; i < n; i++)
            if (A[i] < 0)
                A[i] *= -1, ans++;
        for (int i = pos + 1; i < n - 1; i++)
            if (A[i] > A[i + 1])
            {
                cout << -1 << endl;
                return;
            }
        cout << ans << endl;
    }
}

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    if (!DEBUG)
    {
        freopen((TASK_NAME + ".in").c_str(), "rt", stdin);
        freopen((TASK_NAME + ".out").c_str(), "wt", stdout);
    }

    read();
    solve();

    return EXIT_SUCCESS;
}