#include <bits/stdc++.h>
using namespace std;
const int maxn = 2 * 1000 * 100 + 100;
long long nums[maxn];
long long ans = 0;
long long cou[30];
int main() {
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> nums[i];
    cou[nums[i] + 10]++;
  }
  ans += (cou[0 + 10] * (cou[10] - 1)) / 2;
  for (int i = 1; i < 11; i++) {
    ans += cou[i + 10] * cou[-i + 10];
  }
  cout << ans << endl;
  return 0;
}
