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#include <iostream>
#include <list>
#include <map>

using namespace std;

class Edge;

class Point {
public:
    Edge *left;
    Edge *right;
    int index;
    long long value;

    Point(int i, long long int v) : index(i), value(v) {}

};

class Edge {
public:
    Point *left;
    Point *right;
    int leftIndex;
    int rightIndex;
    long long leftSum;
    long long rightSum;

    Edge(Point *l, Point *r, int lI, int rI) : left(l), right(r), leftIndex(lI), rightIndex(rI) {}

    int length() const {
        return rightIndex - leftIndex;
    }

};

struct EdgeComparator {
    bool operator()(const Edge *edge1, const Edge *edge2) {
        if(edge1->length() == edge2->length()) {
            long long sum1 = edge1->left->value + edge1->right->value;
            long long sum2 = edge2->left->value + edge2->right->value;
            return sum1 > sum2;
        }
        return edge1->length() > edge2->length();
    }
};

int main() {

    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    int n;
    cin >> n;

    list<Edge *> edges;
    map<int, Edge *> map;

    long long leftSum = 0;
    Point *lastPoint = 0;
    long long value;
    for (int i = 0; i < n; i++) {
        cin >> value;
        if (value == 0) {
            continue;
        }


        Point *newPoint = new Point(i, value);
        Edge *edge;
        if (lastPoint != 0) {
            edge = new Edge(lastPoint, newPoint, lastPoint->index, newPoint->index);
            edge->leftSum = leftSum;
            lastPoint->right = edge;
            edges.emplace_back(edge);
        }
        leftSum += value;
        newPoint->left = edge;

        lastPoint = newPoint;
    }

    if (leftSum < 0) {
        cout << -1 << endl;
        return 0;
    }

    long long rightSum = 0;
    for (;;) {
        rightSum += lastPoint->value;
        Edge *edge = lastPoint->left;
        if (edge == 0) {
            break;
        }
        edge->rightSum = rightSum;

        lastPoint = edge->left;
    }

    edges.sort(EdgeComparator());

    int result = 0;
    for (auto const &edge : edges) {
        auto upper = map.upper_bound(edge->leftIndex);
        long long rightSum = edge->rightSum;
        if (upper != map.end()) {
            rightSum -= upper->second->rightSum;
        }

        auto lower = map.lower_bound(edge->leftIndex);
        long long leftSum = edge->leftSum;
        if (lower != map.begin()) {
            --lower;
            leftSum -= lower->second->leftSum;
        }

        if (leftSum >= 0 && rightSum >= 0) {
            map.insert(pair<int, Edge *>(edge->leftIndex, edge));
//            cout << "Removing egde " << edge->leftIndex << " " << edge->rightIndex << " " << leftSum << " " << rightSum
//                 << " " << (edge->left->value + edge->right->value) << endl;
        } else {
            result += edge->length();
//            cout << "Using egde " << edge->leftIndex << " " << edge->rightIndex << " " << leftSum << " " << rightSum
//                    << " " << (edge->left->value + edge->right->value) << endl;

            Point *leftPoint = edge->left;
            Point *rightPoint = edge->right;

            leftPoint->right = rightPoint->right;
            leftPoint->value += rightPoint->value;
        }
    }

    cout << result << endl;

    return 0;
}