#include <fstream>
#define VMAX 7100
#define RVMAX 7000

using namespace std;

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

int V[VMAX], N, SMAX;

int main()
{
    int i, a, j, k;
    fin >>N;
    for (i = 1; i <= N; i++)
    {
        fin >>a;
        V[a]++;
    }
    for (i = 1; i <= RVMAX; i++)
        if (V[i] > 0)
        {
            if (V[i] >= 3) SMAX += ( V[i]*(V[i]-1)*(V[i]-2) )/2  ;
            for (j = i+1, k = i-1; k >= 1 && j <= RVMAX; k--, j++)
                SMAX += V[k] * V[j];
        }
    fout <<SMAX;
    fout.close();
    return 0;
}