#include <fstream>
using namespace std;
ifstream in("medie.in");
ofstream out("medie.out");
int n,v[9001],f[7001],r;
int main()
{ in>>n;
  for(int i=1;i<=n;i++)
  { in>>v[i];
    f[v[i]]++;
  }
  for(int i=1;i<n;i++)
    for(int j=i+1;j<=n;j++)
      if(v[i]%2==v[j]%2)
      { if(v[i]!=v[j])
		   r+=f[(v[i]+v[j])/2];
        else
           r+=f[v[i]]-2;
      }
  out<<r;
  in.close();
  out.close();
  return 0;
}