#include<fstream>
using namespace std;
ifstream fin("medie.in");
ofstream fout("medie.out");
int nr[7005], f[14005], v[7005], n;
long long sol = 0;
int main(){
    fin >> n;
    for( int i = 1; i <= n; i++ ){
        fin >> v[i];
        nr[ v[i] ]++;
    }
    for( int i = 1; i <= n; i++ ){
        for( int j = i + 1; j <= n; j++ ){
            f[ v[i] + v[j] ]++;
        }
    }
    for( int i = 1; i <= n; i++ ){
        sol += ( f[ 2 * v[i] ] - nr[ v[i] ] + 1 );
    }
    fout << sol << "\n";
    return 0;
}