#include <cstdio>
#define DIM 7010

using namespace std;

int a [DIM], rez [DIM], nr, N, i, j, f [DIM], suma;
void solve ()
{
	scanf ("%d\n", &N);
	for (i = 1; i <= N; i++) 
	{
		scanf ("%d\n", &a[i]);
		++ f[ a[i] ];
	}
	
	for (i = 1; i < N; i++)
		for (j = i + 1; j <= N; j++)
		{
			suma = a[i] + a[j];
			if ( (suma & 1) == 0)
			   ++ rez [suma >> 1];
		}
	for (i = 1; i <= N ; i++) 
		nr += rez[a[i]] - f [a[i]] + 1;
		printf ("%d\n", nr);
}
int main ()
{
	freopen ("medie.in", "r", stdin);
	freopen ("medie.out", "w", stdout);
	solve ();
	return 0;
}