#include <fstream>
using namespace std;

const int xn=9000, xv=7000;

int N, V[xn], F[xv];
long long S;

int main(){
	fstream f;

	f.open("medie.in", ios_base::in);
	f >> N;
	for (int i=0; i<N; ++i){
		f >> V[i];
		F[V[i]]++;
	}
	f.close();

	for (int i=0; i<N-1; ++i)
		for (int j=i+1; j<N; ++j)
			S+=((V[i]+V[j])%2==0?F[(V[i]+V[j])/2]:0)-(V[i]==V[j]?2:0);

	f.open("medie.out", ios_base::out);
	f << S << '\n';
	f.close();

	return 0;
}