#include <iostream>
#include <fstream>
using namespace std;

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

int N, S;
int V[9005], fr[14005], ap[14005];

int main()
{
	fin >> N;
	for (int i = 1; i <= N; ++i)
	{
		fin >> V[i];
		++ap[V[i]];
					
		for (int j = 1; j < i; ++j)
			++fr[V[i] + V[j]];
	}
	
	for (int i = 1; i <= N; ++i)
		S = S + fr[V[i] * 2] - (ap[V[i]] - 1);

	fout << S << '\n';
	fin.close();
	fout.close();
}