#include <cmath>
#include <fstream>
using namespace std;
ifstream f("medie.in");
ofstream g("medie.out");

int n,i,j,a[9010],v[7010],app[9010],x;
long sum;
double aux;

int main()
{
    f>>n;
    for (i=1;i<=n;i++)
    {
        f>>a[i];
        app[a[i]]++;
    }
    for (i=1;i<=n;i++)
    {
        for (j=i+1;j<=n;j++)
        {
            aux=(double(a[i])+double(a[j]))/2;
                        //g<<aux<<" "<<floor(aux)<<endl;
            if (aux==floor(aux))
                v[(a[i]+a[j])/2]++;
        }
    }
    //g<<"TUESTIMIRCEA?"<<endl;
    for (i=1;i<=n;i++)
    {
        x=a[i];
        //g<<v[x]<<" ";
        if (v[x]!=0)
            sum=sum+v[x]-app[x]+1;
    }
    g<<sum;
    f.close();
    g.close();
    return 0;
}