#include <iostream>
#include <fstream>
#include <cstring>

using namespace std;

ifstream fin ("medie.in");
ofstream fout ("medie.out");

int v[9000];
int f[7000 * 2 + 1];

int main()
{
    //afisare(rez);

    int n;
    fin >> n;
    for(int i = 0; i < n; i++){
        fin >> v[i];
    }

    for(int i = 0; i < n; i++){
        for(int j = i + 1; j < n; j++){
            if(v[i] != v[j]){
                f[ v[i] + v[j] ]++;
            }
        }
    }

    //mai intai iau elementele distincte
    long long REZ = 0;
    for(int i = 0; i < n; i++){
        REZ += f[ v[i] * 2 ];
    }

    //reciclez vectorul f
    memset(f, 0, sizeof(f));
    //acum va tine nr repetitii ale fiecarui element

    for(int i = 0; i < n; i++){
        f[ v[i] ]++;
    }

    //ma ocup de numerele ce se repeta de mai mult de 2 ori (neinclusiv)
    for(int i = 1; i <= 7000; i++){
        if(f[i] > 2){
            REZ += 1LL * f[i] * (f[i] - 1) * (f[i] - 2) / 2;
        }
    }

    fout << REZ;
    return 0;
}