# include <fstream>
using namespace std;
int a[9010], v[7010], w[7010];
long long nrp, n;
ofstream fout ("medie.out");
void citire ()
{
     int i;
     ifstream fin ("medie.in");
     fin>>n;
     for (i=1;i<=n;i++)
     {
         fin>>a[i];
         v[a[i]]++;
     }
}
void rez ()
{
     int i, j, b;
     for (i=1;i<=n;i++)
         for (j=1;j<=n;j++)
             if (w[a[j]]!=i && i!=j)
             {
                b=2*a[i]-a[j];
                if (b<=7000)
                {
                   if (w[b]!=i && v[b]>0)
                   {
                      if (b==a[j])
                         nrp+=(v[b]-1)*(v[b]-2)/2;
                      else
                         nrp+=v[b]*v[a[j]];
                   }
                   w[a[j]]=i;
                   w[b]=i;
                }
             }
}
int main ()
{
    citire ();
    rez ();
    fout<<nrp;
    return 0;
}