#include <iostream>
#include <vector>
#include <set>

using namespace std;

int _3sum(const vector<int> &a)
{
    const int n = a.size();

    set<int> nums;

    int count = 0;
    for (int i = 0; i < n; i++) {
        for (int j = i + 1; j < n; j++) {
            auto it = nums.find(-a[i] - a[j]);
            if (it != nums.end()) {
                count++;
            }
        }
        nums.insert(a[i]);
    }
    
    return count;
}

int main(void)
{
    int n;
    cin >> n;

    vector<int> a(n);
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }

    cout << _3sum(a) << endl;

    return 0;
}