#include <fstream>
#include <algorithm>

using namespace std;

const int NMAX = 9000;
const int VAL_MAX = 7000;
int v[1 + NMAX];
int frecv[1 + VAL_MAX];

int main()
{
    ifstream in("medie.in");
    ofstream out("medie.out");
    int n, nr_sol = 0;

    in >> n;
    for (int i = 1; i <= n; i++)
    {
        in >> v[i];
        frecv[v[i]]++;
    }

    for (int i = 1; i < n; i++)
    {
        for (int j = i + 1; j <= n; j++)
        {
            if ((v[i] + v[j]) % 2 == 0)
            {
                int mediu = (v[i] + v[j]) / 2;
                nr_sol += frecv[mediu];
                if (v[i] == v[j])
                {
                    nr_sol -= 2;
                }
            }
        }
    }

    out << nr_sol << '\n';

    return 0;
}