#include<fstream>
using namespace std;
int n;
int v[9005];
int contor,poz[7005];
int main()
{
    ifstream fin("medie.in");
    ofstream fout("medie.out");
    fin>>n;
    int i,j;
    for(i=1;i<=n;i++)
      fin>>v[i],poz[v[i]]=1;
    long long a;
    for(i=1;i<=n;i++)
     for(j=i;j<=n;j++)
      { 
       if((v[i]+v[j])%2==0 && i!=j)
           { 
            a=(v[i]+v[j])/2;
            if(poz[a]==1)
            contor++;}
         }
     fout<<contor;
     return 0;
}