#include<fstream>
using namespace std;

int vc[7010],v[9010],n,i,j,nr,fcv[7010];

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

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

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

    nr=0;
    for(i=1;i<=n;i++)
    {
        nr+=vc[v[i]]-fcv[v[i]]+1;
    }



    g<<nr<<"\n";

    f.close();
    g.close();
    return 0;
}