#include <bits/stdc++.h>
using namespace std;
const int OO = (int)2e9;
const double PI = 2 * acos(0.0);
const double EPS = 1e-9;
int dcmp(double a, double b) { return fabs(a - b) <= EPS ? 0 : a > b ? 1 : 2; }
int DI[] = {-1, 0, 1, 0, 1, -1, -1, 1};
int DJ[] = {0, 1, 0, -1, 1, -1, 1, -1};
map<int, long long> m;
int main() {
  int n;
  cin >> n;
  for (int i = 0; i < (int)n; i++) {
    int x;
    cin >> x;
    m[x]++;
  }
  long long ans = 0;
  for (int i = -10; i < 0; i++)
    if (m.count(i) && m.count(abs(i))) ans += (m[i] * m[abs(i)]);
  if (m.count(0)) {
    for (long long t = m[0] - 1; t > 0; t--) ans += t;
  }
  cout << ans << endl;
  return 0;
}
