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#include <iostream>
#include <vector>

int main() {
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(nullptr);
    int n;
    std::cin >> n;
    long long sum = 0;
    std::vector<long long> points;
    points.reserve(n);
    for (int i = 0; i < n; ++i) {
        long long e;
        std::cin >> points[i];
        sum += points[i];
    }
    if (sum < 0) {
        std::cout << -1 << std::endl;
        return 0;
    }
    if (sum == 0) {
        long long start = 0, partialSum = 0, res = 0;
        int i = 0;
        while (i < n) {
            if (partialSum == 0) {
                while (i < n and points[i] == 0) ++i;
                if (i == n) break;
                start = i;
            }
            partialSum += points[i];
            if (partialSum == 0) {
                res += i - start;
            }
            ++i;
        }
        std::cout << res << std::endl;
        return 0;
    }
    long long forcedSegments = 0, firstNegative = -1, partialSum = 0;

    for (int i = 0; i < n; ++i) {
        partialSum += points[i];
        if (partialSum >= 0) {
            if (firstNegative >= 0) {
                forcedSegments += i - firstNegative;
                firstNegative = -1;
            }
        } else {
            if (firstNegative == -1) firstNegative = i;
        }
    }

    firstNegative = -1;
    partialSum = 0;
    for (int i = n - 1; i > -1; --i) {
        partialSum += points[i];
        if (partialSum >= 0) {
            if (firstNegative >= 0) {
                forcedSegments += firstNegative - i;
                firstNegative = -1;
            }
        } else {
            if (firstNegative == -1) firstNegative = i;
        }
    }
    std::cout << (5 * forcedSegments + n) / 6 << std::endl;
    return 0;
}