#include <bits/stdc++.h>
using namespace std;
inline int max(int a, int b) { return a > b ? a : b; }
inline int min(int a, int b) { return a < b ? a : b; }
map<int, int> mp;
int main() {
  int n, x;
  cin >> n;
  mp.clear();
  for (int i(0); i < (n); ++i) {
    cin >> x;
    ++mp[x];
  }
  long long ans(0);
  for (map<int, int>::iterator it = mp.begin(); it != mp.end() && it->first < 0;
       ++it)
    ans += (long long)it->second * (long long)mp[-it->first];
  ans += (long long)mp[0] * ((long long)mp[0] - 1ll) / 2ll;
  cout << ans << endl;
  return 0;
}
