#include <stdio.h>
#define NMAX 9001

int n,v[NMAX],f[NMAX],fm[NMAX],k,i,j;

int main()
{
  freopen("medie.in","r",stdin);
  freopen("medie.out","w",stdout);

  scanf("%d", &n);
  for (i=1;i<=n;i++)
	{
	 scanf("%d", &v[i]);
	 f[v[i]]++;
	}
  for (i=1;i<n;i++)
  for (j=i+1;j<=n;j++)
	 if (!((v[i]+v[j])&1)) fm[(v[i]+v[j])>>1]++;
  for (i=1;i<=n;i++)
	if (fm[v[i]])
		k+=fm[v[i]]-f[v[i]]+1;
  printf("%d", k);
  return 0;
}