#include <bits/stdc++.h>
using namespace std;
int main() {
  unsigned long long n, ans = 0, a[30], cur, deg = 20, countOfPair = 1;
  cin >> n;
  memset(a, 0, sizeof(unsigned long long) * 30);
  for (int i = 0; i < n; ++i) {
    cin >> cur;
    ++a[cur + 10];
  }
  for (int i = 0; i < 10; ++i) {
    ans += a[i] * a[deg + i];
    deg -= 2;
  }
  if (a[10] != 0 && a[10] != 1) {
    if (a[10] == 2)
      ans += 1;
    else {
      for (unsigned int i = 3; i <= a[10]; ++i) {
        countOfPair = countOfPair + i - 1;
      }
      ans += countOfPair;
    }
  }
  cout << ans;
  cin >> n;
  return 0;
}
