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#include <iostream>
#include <vector>
#include <unordered_map>

#define N 500002

using namespace std;

vector<int> points;
unordered_map<int, int> pointsMap;

long long prefixSumPower[N] = {0};
long long minDistance[N] = {0};

int main() {
    ios::sync_with_stdio(false);

    int n;
    cin >> n;
    for (int i = 0; i < n; i++) {
        int a;
        cin >> a;
        if(a != 0) {
            pointsMap[points.size()] = i;
            points.push_back(a);
        }
    }

    if (points.empty()) {
        cout << 0 << endl;
        exit(0);
    }

    prefixSumPower[0] = 0;
    for (int i = 0; i < points.size(); i++) {
        prefixSumPower[i+1] = points[i] + prefixSumPower[i];
    }

    minDistance[0] = 0;
    minDistance[1] = (points[0] < 0) ? INT64_MAX : 0;

    for (int i = 2; i <= points.size(); ++i) {
        long long mDist = INT64_MAX;
        if (points[i-1] > 0) {
            mDist = minDistance[i-1];
        }
        for (int j = 1; j < i; ++j) {
            if (minDistance[j-1] == INT64_MAX) {
                continue;
            }
            long long power = prefixSumPower[i] - prefixSumPower[j-1];
            if (power < 0) {
                continue;
            }
            long long dist = pointsMap[i-1] - pointsMap[j-1] + minDistance[j-1];
            mDist = min(mDist, dist);
        }
        minDistance[i] = mDist;
    }

    long long res = minDistance[points.size()];
    res = (res == INT64_MAX) ? -1 : res;
    cout << res << endl;

    return 0;
}