#include<fstream>
#define g 9010
using namespace std;
int a[g],c[g],frec[g];
int main()
 {
  ifstream be ("medie.in");
  ofstream ki ("medie.out");
  int i,j,n,sz=0,k;
  float l,x,y;
  be>>n;
  for (i=1;i<=n;i++)
     {
      be>>a[i];
      c[a[i]]=1;
     }
  be.close();
  for (i=1;i<n;i++)
     {
      frec[a[i]]++;
      for (j=i+1;j<=n;j++)
	 {
	  x=a[i]; y=a[j];
	  k=(x+y)/2; l=(x+y)/2;
	  if (k==l)
	    c[k]++;
	 }
     }
  frec[a[n]]++;
  for (i=1;i<=n;i++)
     if (c[a[i]])
       sz+=c[a[i]]-frec[a[i]];
  ki<<sz<<'\n';
  ki.close();
  return 0;
 }