#include <fstream>
#include <algorithm>

using namespace std;

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

int N, vf[7005];
long long sol;

int main()
{
    fin >> N;

    int x;
    for(int i = 1; i <= N; i++)
        {
            fin >> x;
            vf[x]++;
        }

    for(int i = 1; i <= 7000; i++)
        if(vf[i] >= 1)
        {
            if(vf[i] >= 3)
                sol += (vf[i]) * (vf[i] - 1) * (vf[i] - 2) / 2;

            for(int j = 1; i - j >= 1 && i + j <= 7000; j++)
                if(vf[i - j] >= 1 && vf[i + j] >= 1)
                    sol += vf[i] * vf[i - j] * vf[i + j];
        }

    fout << sol << '\n';

    return 0;
}