#include <cstdio>
#include <iostream>
#include <set>
#include <climits>
#include <map>
#include <algorithm>
#include <list>
#include <vector>
#include <utility>

using namespace std;

int main() {
    freopen("nrtri.in", "r", stdin);
    freopen("nrtri.out", "w", stdout);

    ios::sync_with_stdio(false);
    cin.tie(0);

    int n, v[805];
    cin >> n;
    for (int i = 0; i < n; i++) {
        cin >> v[i];
    }
    v[n] = INT_MAX;
    sort(v, v + n);
    int ans = 0;
    for (int i = 0; i < n - 2; i++) {
        for (int j = i + 1; j < n - 1; j++) {
            int t = v[i] + v[j], st = j + 1, dr = n - 1;
            while (st <= dr) {
                int mij = (st + dr) / 2;
                if (v[mij] == t) {
                    st = mij;
                    break;
                }
                if (v[mij] < t) {
                    st = mij + 1;
                } else {
                    dr = mij - 1;
                }
            }
            while (v[st] > t) {
                st--;
            }
            while (v[st] == v[st + 1]) {
                st++;
            }
            ans += st - j;
        }
    }
    cout << ans;

    return 0;
}