#include <bits/stdc++.h>
using namespace std;

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

int n;
int N[9004];
long long fr_suma[14004];
long long fr[7004];
long long rez;
int main()
{
    fin >> n;
    for (int i = 1; i <= n; i++)
    {
        fin >> N[i];
        fr[N[i]]++;
    }
    for (int i = 1; i <= n - 1; i++)
    {
        for (int j = i + 1; j <= n; j++)
        {
            fr_suma[(N[i] + N[j])]++;
        }
    }
    N[0] = -1;
    for (int i = 1; i <= n; i++)
    {
        rez += (fr_suma[(2 * N[i])] - fr[N[i]] + 1);
    }
    fout << rez;
    return 0;
}