import java.util.Scanner;
import java.util.Map;
import java.util.HashMap;

public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int res= 0;
        int n = sc.nextInt();
        Map<Integer,Integer> hash = new HashMap<>();
        for(int i = 1;i<=n;i++){
            int t = sc.nextInt();
            int ft = -t;
            // 直接找里面有没有对应的相反数
            if(hash.get(ft) != null){
                res += hash.get(ft);
            }
            hash.put(t,hash.getOrDefault(t,0)+1);
        }
        System.out.println(res);
    }
}