暴力

#include <iostream>

using namespace std;

const int N = 550;
int n;
int a[N];

int main(){
    cin>>n;
    for (int i = 0;i<n;i++) cin>>a[i];
    int cnt = 0;
    for (int i = 0;i<n;i++){
        for (int j = i;j <n;j++){
            int tmp = -a[i];
            if (tmp == a[j]) cnt++;
        }
    }
    cout<<cnt<<endl;

    return 0;
}