#pragma GCC optimize 03

#include <bits/stdc++.h>

using namespace std;

const int m = 2e4 + 1;

int main() {
    int n;
    cin >> n;
    vector<int> a(n);
    for (auto &el : a) cin >> el;
    vector<int> cnt(m);
    for (auto el : a) cnt[el]++;

    long long ans = 0;
    for (int i = 0; i < n; ++i) {
        cnt[a[i]]--;
        for (int j = 0; j < i; ++j) {
            ans += cnt[a[i] + a[j]];
        }
    }
    cout << ans << '\n';
    return 0;
}