#define _CRT_SECURE_NO_WARNINGS
#define _SILENCE_CXX17_C_HEADER_DEPRECATION_WARNING
#include <bits/stdc++.h>
using namespace std;
int main()
{
    int n;
    cin >> n;
    vector<int> v1(n), v2;
    v2.reserve(n * (n - 1) / 2);
    for (auto &x : v1)
        cin >> x;
    for (int i = 0; i < n; i++)
        for (int j = 0; j < i; j++)
            v2.push_back(v1[i] + v1[j]);
    sort(v2.begin(), v2.end());
    cout << count_if(v1.begin(), v1.end(), bind(binary_search<decltype(v2)::iterator, decltype(v2)::value_type>, v2.begin(), v2.end(), placeholders::_1));
    return 0;
}