# include <stdio.h>
int N, ap[9001],a[9001],Ap[9001];
int main(){
  int k,i,j;
  freopen("medie.in", "r", stdin);
  freopen("medie.out", "w", stdout);
  scanf("%d",&N);
  for (i=1;i<=N;++i) {
		scanf("%d",&a[i]);
		ap[a[i]]++;
	}
  for (i=1;i<N;++i){
	for (j=i+1;j<=N;++j)
	  if (!((a[i]+a[j])&1)) Ap[(a[i]+a[j])>>1]++;
   }
  for (i=1,k=0;i<=N;++i)
	if (Ap[a[i]]) k+=Ap[a[i]]-ap[a[i]]+1;
  printf("%d",k);
  return 0;
}