#include <fstream>
using namespace std;

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

int n,v[9003],s[14003],ap[9003],nr;
int main()
{

    f>>n;
    for(int i=1;i<=n;++i)f>>v[i],ap[v[i]]++;
    for(int i=1;i<n;++i)
        for(int j=i+1;j<=n;++j)s[v[i]+v[j]]++;
    for(int i=1;i<=n;++i)
    {
        nr+=s[2*v[i]]-ap[v[i]]+1;
    }
    g<<nr<<'\n';
    return 0;
}