#include<fstream.h>
#define g 10000
using namespace std;
float a[g],c[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-1;i++)
     for (j=1;j<=n;j++)
       { x=a[i];y=a[j];
	 k=(x+y)/2; l=(x+y)/2;
	if (c[k]&&j!=i&&k==l)
	  sz++;        }
  ki<<sz<<'\n';
  ki.close();
  return 0;
 }