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#include <cstdio>
#include <cassert>
#include <algorithm>
#include <functional>
#include <unordered_map>

using namespace std;

int n;
int j;

typedef struct {
        long long val;
        long long cost;
        long long sumval;
        long long sumcost;
} node;

unordered_map<std::string,node> m;

const int max_n = 500001;

long len[max_n];

void print_len()
{
        for(int i = 1; i < j; ++i) {
                printf("(%d), (%ld)\n", i, len[i]);
        }
        printf("\n");
}

string make_key(int a, int b)
{
        char s[25];
        sprintf(s, "%d %d", a, b);
        return string(s);
}

void print_map()
{
        for (auto& x: m) {
            printf("%s - val %lld, cost %lld, sumval %lld, sumcost %lld\n", x.first.c_str(), x.second.val, x.second.cost, x.second.sumval, x.second.sumcost);
        }
}

void print_node(node nd)
{
        printf("val %lld, cost %lld, sumval %lld, sumcost %lld\n", nd.val, nd.cost, nd.sumval, nd.sumcost);
}

node calc(int a, int b)
{
        node nd1, nd2;
        bool found = false;
        node best;

        best.cost = -1;

        for(int i = a; i < b; ++i) {
                auto it = m.find( make_key(a, i) );

                if ( it == m.end() ) {
                        nd1 = calc(a, i);
                        m.insert( pair<string,node>(make_key(a, i), nd1) );
                } else {
                        nd1 = it->second;
                }

                it = m.find( make_key(i+1, b) );
                if ( it == m.end() ) {
                        nd2 = calc(i+1, b);
                        m.insert( pair<string,node>(make_key(i+1, b), nd2) );
                } else {
                        nd2 = it->second;
                }

                if (nd1.val >= 0 && nd2.val >= 0) {
                        found = true;
                        if (best.cost == -1 || best.cost > nd1.cost + nd2.cost) {
                                best.cost = nd1.cost + nd2.cost;
                                best.val = nd1.val + nd2.val;
                        }
                }
        }
        best.sumval = nd1.sumval + nd2.sumval;
        best.sumcost = nd1.sumcost + len[b-1];
        if (!found) {
                best.cost = best.sumcost;
                best.val = best.sumval;
        }
        return best;
}

int main()
{       
        node nd;
        int prev=1;
        long k;
        long long sumval=0;
        bool is_factory = false;

        scanf("%d\n", &n);
        nd.cost = 0;
        nd.sumcost = 0;
        j = 0;
        for(int i=1; i <= n; ++i) {
                scanf("%ld", &k);
                if (k == 0) {
                        prev++;
                        continue;
                }
                j++;
                if (j > 1) {
                        len[j-1] = prev;
                }
                prev = 1;
                nd.val = k;
                nd.sumval = k;
                sumval += k;
                if (k < 0) {
                        is_factory = true;
                }
                m.insert( pair<string,node>(make_key(j, j), nd) );
        }

        if (sumval < 0) {
                printf("-1\n");
                return 0;
        }

//        print_len();
//        print_map();

        nd = calc(1, j);

//        print_map();
//        print_node(nd);

        if (nd.cost > 0) {
                printf("%lld\n", nd.cost);
        } else {
                if (is_factory) {
                        printf("-1\n");
                } else {
                        printf("0\n");
                }
        }
        
        return 0;
}