#include <bits/stdc++.h>
using namespace std;
long long n, i, a, b, x, y, o, z, p, q, sum, c;
int main() {
  cin >> n;
  vector<long long> v;
  for (i = 0; i < n; i++) {
    cin >> x;
    if (x == 0)
      o++;
    else
      v.push_back(x);
  }
  sort(v.begin(), v.end());
  a = 0;
  b = v.size() - 1;
  sum += o * (o - 1) / 2;
  if (v.size() > 1) {
    while (a < b) {
      if (v[a] > 0 || v[b] < 0)
        break;
      else {
        if (v[a] + v[b] < 0)
          a++;
        else if (v[a] + v[b] > 0)
          b--;
        else {
          p = 1;
          q = 1;
          x = v[a];
          a++;
          y = v[b];
          b--;
          while (1) {
            if (v[a] == x) {
              p++;
              a++;
            } else
              break;
          }
          while (1) {
            if (v[b] == y) {
              q++;
              b--;
            } else
              break;
          }
          sum += p * q;
        }
      }
    }
  }
  cout << sum;
}
