#include<fstream>
using namespace std;
ifstream f("medie.in");
ofstream g("medie.out");
int n, nr, v[9001];
void citire()
{
    int i,j,k;
    float m;
    f>>n;
    for(i=1;i<=n;i++)f>>v[i];
    for(i=1;i<n;i++)
    {
        for(j=i+1;j<=n;j++)
        {
              m=(v[i]+v[j])/2.;
              for(k=1;k<=n;k++)
              {
                  if(k!=i&&k!=j&&m==v[k])nr++;
              }
        }

    }
    g<<nr;
}
int main()
{
    citire();
    f.close();
    g.close();
    return 0;
}