#include <iostream>
#include <map>
using namespace std;
map<int, int> mp;
int main(){
    int n;  cin >> n;
    int cnt = 0;
    for(int i = 0; i < n; i ++ ){
        int tmp;    cin >> tmp;
        cnt += mp[-tmp];
        mp[tmp] ++ ;
    }
    cout << cnt << endl;
    return 0;
}