#include <stdio.h>
#define DIM 9002

int n,i,t,j;
int v[DIM];
int A[DIM];
int M[DIM];

int main(){
  FILE *f = fopen("medie.in","r");
  fscanf(f,"%d", &n);
  for (i=1;i<=n;i++)
    fscanf(f,"%d",&v[i]);
  fclose(f);

  for (i=1;i<n;i++){
    A[v[i]]++;
    for (j=i+1;j<=n;j++)
      if (!((v[i]+v[j])&1))
	M[(v[i]+v[j])>>1]++;
  }
  A[v[n]]++;
  for (i=1;i<=n;i++)
    if (M[v[i]]!=0)
      t = t + (M[v[i]]-A[v[i]]+1);
  FILE *g = fopen("medie.out","w");
  fprintf(g,"%d",t);
  fclose(g);

  return 0;
}