#include <fstream>
using namespace std;

int v[9001],nr[7001],fv[7001],s;
int main() {
	ifstream f("medie.in");
	ofstream g("medie.out");
	int n,i,j;
	f>>n;
	for (i=1; i<=n; i++) {
		f>>v[i];
		nr[v[i]]++;
	}
	for (i=1; i<n; i++)
		for (j=i+1; j<=n; j++)
			if (!((v[i]+v[j])%2))
				fv[(v[i]+v[j])/2]++;
			
	for (i=1; i<=n; i++)
		if (fv[v[i]])
			s+=(fv[v[i]]-nr[v[i]]+1);
		
	g<<s<<'\n';
	g.close();
	return 0;
}