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#include <iostream>
#include <vector>
using namespace std;

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);

  int n; cin >> n;
  vector<int> fa(n);
  vector<int> pos;

  int ns = 0;
  int gs = 0;
  
  for(int i = 0; i < n; i++) {
    int a; cin >> a;
    fa[i] = a;
    if(a < 0) {
      pos.push_back(i);
      ns += -a;
    }
    else gs += a;
  }

  if(ns > gs) {
    cout << "-1\n";
    return 0;
  }
  if(ns == 0) {
    cout << "0\n";
    return 0;
  }

  long long ans = 0;
  for(int ip: pos) {
    int i = ip;
    long long s = 0;
    while(s < abs(fa[ip])) {
      if(i == n - 1) break;
      s += fa[++i];
    }
    int dist1;
    if(s < abs(fa[ip])) dist1 = 10000000;
    else dist1 = i - ip;

    s = 0;
    i = ip;
    while(s < abs(fa[ip])) {
      if(i == 0) break;
      s += fa[--i];
    }
    int dist2;
    if(s < abs(fa[ip])) dist2 = 10000000;
    else dist2 = ip - i;

    ans += min(dist1, dist2);
  }

  cout << ans << "\n";

  return 0;
}