#include <fstream>
using namespace std;

ifstream f("medie.in");
ofstream g("medie.out");

int x,y,a[9005],frq[9005],n,i,j,m;
long long cont;

int main()
{
    f>>n;
    for (i=1;i<=n;i++)
       {
        f>>a[i];
        frq[a[i]]++;
       }
    for (i=1;i<=n;i++)
     for (j=i+1;j<=n;j++)
     {
         m=(a[i]+a[j]);
         if (m%2==0)
         {
             m/=2;
             if (m==a[i])
              cont--;
             if (m==a[j])
              cont--;
            cont+=frq[m];
         }
     }

    g<<cont;
    return 0;
}