#include <iostream>
#include <fstream>

using namespace std;

ifstream f ("medie.in");
ofstream g ("medie.out");

const int NMAX = 9000 + 1, VMAX = 7000 + 1;

int n;
int v[NMAX], fr[VMAX * 2];

void citeste () {
    f >> n;
    for (int i = 1; i <= n; i++) f >> v[i], fr[v[i] * 2]++;
}

void rezolva () {
    int sol = 0;
    for (int i = 1; i < n; i++)
    for (int j = i + 1; j <= n; j++) {
        sol += fr[v[i] + v[j]];
        if (v[i] == v[j]) sol -= 2;
    }
    g << sol << '\n';
}

int main () {
    citeste ();
    rezolva ();
    return 0;
}