#include <fstream>
using namespace std;
ifstream f ("medie.in");
ofstream g ("medie.out");
unsigned long long nr;
int n,i,j,v[7001],t,maxim;
int main()
{
    f>>n;
    for(i=1;i<=n;++i)
    {
        f>>t;
        v[t]++;
        if(t>maxim) maxim=t;
    }
    for(i=1;i<=maxim;++i)
    {
        nr=nr+(v[i]-2)*(v[i]-1)/2*v[i];
        if(v[i])
        for(j=1;j<i;++j)
        {
            nr=nr+v[j]*v[2*i-j];
        }
    }
    g<<nr;
    return 0;
}