#include <bits/stdc++.h>
using namespace std;
map<long long, long long> m;
long long n, p;
int main() {
  cin >> n;
  while (cin >> n) m[n]++;
  for (n = 1; n < 11; n++) p += m[n] * m[n * -1];
  cout << p + m[0] * (m[0] - 1) / 2;
}
