#include <stdio.h>

int n,v[9001],m[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]);
	 
	}
	
  for (i=1;i<n;i++)
      for (j=i+1;j<=n;j++)
	           if ((m[i]+v[j])%2==0) m[(v[i]+v[j])/2]++;
	           
  for (i=1;i<=n;i++)
               	if (m[v[i]]) k=k+m[v[i]];
		
  printf("%d", k);
  return 0;
}