#include <stdio.h>

int n,v[9001],m[9001],u[9001],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]);
	 u[v[i]];
	 
	}
	
  for (i=1;i<n;++i)
      for (j=i+1;j<=n;j++)
	           if ((v[i]+v[j])&1!=0) m[(v[i]+v[j])>>1]++;
	           
  for (i=1;i<=n;++i)
               	if (m[v[i]]>0){
                                k=k+m[v[i]]-u[v[i]];
		                        k++;
                                }
  printf("%d", k);
  return 0;
}