#include <fstream>
using namespace std;
ifstream fin ("medie.in");
ofstream fout ("medie.out");
int N, sol, V[9010], fr[9010];

int main()
{
    fin >> N;
    for (int i=1; i<=N; i++) fin >> 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 x = V[i] + V[j];
                fr[x / 2] += 1;
                if (x / 2 == V[i]) sol -= 1;
                if (x / 2 == V[j]) sol -= 1;
            }
        }
    }

    for (int i=1; i<=N; i++)
    {
        sol += fr[V[i]];
    }

    fout << sol << '\n';
    fout.close();
    return 0;
}