#include <fstream>
using namespace std;

ifstream is("medie.in");
ofstream os("medie.out");

int N, Frv[14001], Frv2[15001], Max(-1), X[9001];
int Solution;

int main()
{
    is >> N;
    for ( int i = 1; i <= N; ++i )
    {
        is >> X[i];
        Frv2[2*X[i]]++;
    }
    for ( int i = 1; i <= N; ++i )
        for ( int j = i + 1; j <= N; ++j )
        {
            Solution += Frv2[(X[i]+X[j])];
            if ( X[i] == X[j] )
                Solution -= 2;
        }
    os << Solution;
    return 0;
}