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#include<cstdio>
#include<iostream>
#include<algorithm>
#include<string>
#include<vector>
#include<queue>

using namespace std;

typedef vector<int> VI;
typedef long long LL;

#define FOR(x, b, e) for(int x=b; x<=(e); ++x)
#define FORD(x, b, e) for(int x=b; x>=(e); --x)
#define REP(x, n) for(int x=0; x<(n); ++x)
#define VAR(v, n) __typeof(n) v = (n)
#define ALL(c) (c).begin(), (c).end()
#define SIZE(x) ((int)(x).size())
#define FOREACH(i, c) for(VAR(i, (c).begin()); i != (c).end(); ++i)
#define PB push_back
#define ST first
#define ND second

struct Vert {
    Vert *prev, *next;
    int val, dist, distsum = 0;
    LL sum;
    Vert(int v, int d): val(v), dist(d), sum(v) {
        prev = next = 0;
    } 
};

struct List {
    Vert *begin, *end;
    void add(int v, int d) {
        Vert *tmp = new Vert(v, d);
        if(begin == 0) begin = end = tmp;
        else {
            tmp->sum += end->sum;
            end->next = tmp;
            tmp->prev = end;
            end = tmp;
        }
    }
};

void printList(List *l) {
    Vert *b = l->begin;
    while(b != 0) {
        printf("(%d, %d) -> ", b->val, b->dist);
        b = b->next;
    }
    printf("\n");
}

int main() {
	ios_base::sync_with_stdio(0);
    int n;
    cin >> n;
    List *list = new List;
    LL sum = 0;
    int prev = 0;
    REP(i, n) {
        int a;
        cin >> a;
        if(a == 0) continue;
        sum += a;
        list->add(a, i - prev);
        prev = i;
    }
    if(sum < 0) {
        cout << "-1\n";
    } else {
        int res = 0;
        Vert *b = list->begin;
        while(true) {
            //printList(list);
            Vert *f = b;
            while(f != 0 && f->val >= 0) {
                if(f->next != 0) f->next->distsum = f->next->dist + f->distsum;
                f = f->next;
            }
            if(f == 0) {
                cout << res << '\n';
                return 0;
            }
            while(b->prev != 0 && f->sum - b->sum + b->val < 0) b = b->prev;
            Vert *e = f;
            while((e->sum - b->sum + b->val) < 0) {
                if(e->next != 0) e->next->distsum = e->next->dist + e->distsum;
                e = e->next;
            }
            while((e->sum - b->sum) >= 0 && b != f && b->next != e) {
                if(b->next != 0) b->next->distsum = b->next->dist + b->distsum;
                b = b->next;
            }
            Vert *tb = b->next;
            Vert *te = e;
            while(tb != f->next) {
                while(te != 0 && te->sum - tb->sum + tb->val < 0 && te->distsum - tb->distsum <= e->distsum - b->distsum) {
                    if(te->next != 0) te->next->distsum = te->next->dist + te->distsum;
                    te = te->next;
                }
                while(te != 0 && (te->sum - tb->sum) >= 0 && tb != f) tb = tb->next;
                if(te != 0 && te->distsum - tb->distsum <= e->distsum - b->distsum) {
                    b = tb;
                    e = te;
                }
                tb = tb->next;
            }
            res += e->distsum - b->distsum;
            b->val += e->sum - b->sum;
            b->next = e->next;
            b->sum = e->sum;
            if(b->next != 0) b->next->prev = b;
        }
    }
    return 0;
}