#include <fstream>
#include <cmath>

using namespace std;

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

int n, sum;
int v[9002], a[7002], w[9002];
double aux;

int main()
{
    fin >> n;
    for (int i = 1; i <= n; ++i)
    {
        fin >> v[i];
        ++a[v[i]];
    }
    for (int i = 1; i <= n; ++i)
    {
        for (int j = i + 1; j <= n; ++j)
        {
            aux = double(v[i] + v[j]) / 2;
            if (aux == floor(aux))
                ++w[(v[i] + v[j]) >> 1];
        }
    }
    sum = 0;
    for (int i = 1; i <= n; ++i)
    {
        if (w[v[i]] != 0)
            sum += w[v[i]] - a[v[i]] + 1;
    }
    fout << sum;
    fin.close();
    fout.close();
    return 0;
}