#include<stdio.h>
FILE*fin=fopen("medie.in","r");
FILE*fout=fopen("medie.out","w");
int main()
{
  int n,nr[10000],i,j,v[10000],rez=0;
  fscanf(fin,"%d",&n);
  for(i=0;i<=7000;i++)
    nr[i]=0;
  for(i=1;i<=n;i++)
  {
    fscanf(fin,"%d",&v[i]);
    nr[v[i]]++;
  }
  fclose(fin);
  for(i=1;i<=n;i++)
  {
    for(j=1;j<v[i];j++)
      if(2*v[i]-j<=7000) rez+=nr[j]*nr[2*v[i]-j];
    if(nr[v[i]]>2) rez+=(nr[v[i]]-1)*(nr[v[i]]-2)/2;
  }
  fprintf(fout,"%d",rez);
  fclose(fout);
  return 0;
}