#include <stdio.h>
#include <vector>
using namespace std;

vector <int> v;
vector <int> ap;
int n, i, j;
int sol, mijl;

int main()
{
	FILE *f = fopen ("medie.in","r");
	FILE *g = fopen ("medie.out","w");
	fscanf(f,"%d", &n);

	v.push_back(0);
	
	for (i=0; i<n; ++i)
	{
		fscanf(f,"%d", &j);
		v.push_back(j);
	}
	
	v.resize(16000);
	ap.resize(16000);
	
	for (i=1; i<=n; ++i)
		for (j=i+1; j<=n; ++j)
		{
			mijl = v[i] + v[j];
			ap[mijl] ++;
			if (mijl == v[i] * 2)
				sol --;  
		}
     
	for (i=1; i<=n; ++i)
	{
		sol += ap[v[i]*2];
		ap[v[i]*2] --;
	}
	
	fprintf (g,"%d", sol);
	
	fclose(g);
	fclose(f);
	return 0;     
}