#include <fstream>

using namespace std;

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

int n, v[7005], frecvrez[7005], frecv[7005];

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

void creare_frecvrez() {
    for (int i = 0; i < n; ++i)
        for (int j = i + 1; j < n; ++j)
            if (v[i] % 2 == v[j] % 2)
                frecvrez[(v[i] + v[j]) / 2]++;
}

void afisare() {
    long long sol = 0;
    for (int i = 0; i < n; ++i)
        if (frecvrez[v[i]])
            sol += frecvrez[v[i]] - frecv[v[i]] + 1;
    g << sol;
}

int main() {
    citire();
    creare_frecvrez();
    afisare();
    return 0;
}