#include <fstream>
using namespace std;

const int xn=9000, xv=7000;

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

int main(){
	int i, j, m;
	fstream f;

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

	for (i=0; i<N-1; i++)
		for (j=i+1; j<N; j++){
			m=V[i]+V[j];
		  if (m%2==0){
				S+=F[m>>2];
				if (V[i]==V[j])
					S-=2;
		  }
		}

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

	return 0;
}