#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,aux;
  be>>n;
  for (i=1;i<=n;i++)
     {
      be>>a[i];
     }
  be.close();
# define div2(x) (!((x)&1) )
  for (i=1;i<n;i++)
     {
	++frec[a[i]];
      for (j=i+1;j<=n;j++)
	 {
	  if (div2(aux=a[i]+a[j]))
	    ++c[aux>>1];
	 }
     }
  frec[a[n]]++;
  for (i=1;i<=n;i++)
     if (c[a[i]])
       sz+=c[a[i]]+1-frec[a[i]];
  ki<<sz<<'\n';
  ki.close();
  return 0;
 }