// rezolvare completa

#include <fstream>

using namespace std;

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

int v[10000],frecv[10000],f[10000],i,j,n,suma;


int main()
{
    in >> n;
    for (i=1;i<=n;i++)
    {
        in >> v[i];
        f[v[i]]++;
    }
    for (i=1;i<=n-1;i++)
    {

        for (j=i+1;j<=n;j++)
        if ((v[i]+v[j])%2==0)
            frecv[(v[i]+v[j])/2]++;
    }

    for (i=1;i<=n;i++) suma += frecv[v[i]] - f[v[i]] + 1;
    out << suma << "\n";
}