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#include <iostream>
#include <vector>

using namespace std;

int main()
{
    int n;
    cin >> n;

    vector<pair<long,int>> domki;
    vector<pair<long,int>>::iterator i;

    long dpop = 0;
    int droga = 1;
    long long bilans = 0;
    while (n--) {
        long d;
        cin >> d;
        if (d == 0) {
            droga++;
        }
        else {
            bilans += d;
            if (dpop != 0)
                domki.push_back(make_pair(dpop,droga));
            dpop = d;
            droga = 1;
        }
    }
    domki.push_back(make_pair(dpop, 0));

    //for (i = domki.begin(); i != domki.end(); ++i) {
    //    cout << (*i).first << "(" << (*i).second << ")" << " ";
    //}
    //cout << endl;
    //cout << "Bilans:" << bilans << " Ile:" << domki.size() << endl;

    int suma = 0;
    if (bilans < 0) {
        suma = -1;
        cout << suma << endl;
        return 0;
    }

    long long total = 0;
    for (i = domki.begin(); i != domki.end(); ++i) {
        total = total + (*i).first;
        while (total < 0 && i + 1!= domki.end()) {
            suma += (*i).second;
            total = total + (*(i+1)).first;
            (*i).first = (*i).first + (*(i + 1)).first;
            (*i).second = (*(i + 1)).second;
            domki.erase(i + 1);
        }
        while (i != domki.begin() && i + 1 == domki.end() && (*i).first <= 0) {
            suma += (*(i-1)).second;
            (*(i - 1)).second = 0;
            (*(i-1)).first = (*i).first + (*(i - 1)).first;
            i--;
            domki.erase(i+1);
        }
    }
    //cout << "Total:" << total << endl;

    //cout << "Suma 1:" << suma << " Ile:" << domki.size() << endl;
    //for (i = domki.begin(); i != domki.end(); ++i) {
    //    cout << (*i).first << "(" << (*i).second << ")" << " ";
    //}
    //cout << endl;

    total = 0;
    int ip = 0;
    while (ip < domki.size()) {
        //cout << "ip:" << ip << " first:" << domki[ip].first << " size:" << domki.size() << endl;
        if (domki[ip].first >= 0) {
            total += domki[ip].first;
            ip++;
        }
        else {
            if (total > 0 && domki[ip].second < domki[ip - 1].second) {
                //cout << "prawy" << endl;
            }
            else {
                //cout << "lewy" << endl;
                ip--;
                total -= domki[ip].first;
            }
            domki[ip].first += domki[ip + 1].first;
            suma += domki[ip].second;
            domki[ip].second = domki[ip + 1].second;
            domki.erase(domki.begin() + ip + 1);
        }
        //cout << "Suma po:" << suma << " Ile:" << domki.size() << endl;
        //for (i = domki.begin(); i != domki.end(); ++i) {
        //    cout << (*i).first << "(" << (*i).second << ")" << " ";
        //}
        //cout << endl;
        //cout << endl;
    }
    //cout << "Total:" << total << endl;

    //cout << "Suma 2:" << suma << " Ile:" << domki.size() << endl;
    //for (i = domki.begin(); i != domki.end(); ++i) {
    //    cout << (*i).first << "(" << (*i).second << ")" << " ";
    //}
    //cout << endl;

    cout << suma << endl;

    //for (i = domki.begin(); i != domki.end(); ++i) {
    //    cout << (*i).first << "(" << (*i).second << ")" << " ";
    //}
    //cout << endl;

    return 0;
}