#include <fstream>

using namespace std;

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

const int nmax= 9000;
const int vmax= 7000;

int v[nmax+1], u[2*vmax+1];

int main(  ) {
    int n;
    fin>>n;
    for ( int i= 1; i<=n; ++i ) {
        fin>>v[i];
        ++u[v[i]*2];
    }

    int sol= 0;
    for ( int i= 1; i<=n-1; ++i ) {
        for ( int j= i+1; j<=n; ++j ) {
            sol+= u[v[i]+v[j]];
            if ( v[i]==v[j] ) {
                sol-= 2;
            }
        }
    }

    fout<<sol<<"\n";

    return 0;
}