#define MAX_V 7000
#define MIN_V 1

#include <fstream>
#include <cstdint>
using namespace std;

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

int64_t F[MAX_V + 1];

int main()
{
    int n;
    fin >> n;
    
    for (int i = 1; i <= n; ++i)
    {
        int x;
        fin >> x;
        
        ++F[x];
    }
    
    int64_t res = 0;
    
    for (int i = MIN_V; i <= MAX_V; ++i)
    {
        if (F[i] > 2)
        {
            res += (F[i] * (F[i] - 1) * (F[i] - 2)) / 2;
        }
        else
        {
            for (int d = 1; ((i - d) >= MIN_V) && ((i + d) <= MAX_V); ++d)
            {
                res += F[i] * ((F[i - d] < F[i + d]) ? F[i - d] : F[i + d]);
            }
        }
    }
    
    fout << res;
    
    fin.close();
    fout.close();
    return 0;
}