#include <fstream>

using namespace std;

const int Nmax = 9005;

int A[Nmax], Count[Nmax];

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

    int N;
    fin >> N;

    int Sol = 0;
    for (int i = 1; i <= N; i++)
    {
        fin >> A[i];

        for (int j = 1; j < i; j++)
        {
            if ((A[i] + A[j]) % 2 == 0)
            {
                Count[(A[i] + A[j]) / 2]++;

                if (A[i] == A[j]) Sol -= 2;
            }
        }
    }

    for (int i = 1; i <= N; i++) Sol += Count[A[i]];

    fout << Sol << "\n";

    fin.close();
    fout.close();
}