#include <fstream>
#include <math.h>
using namespace std;
ifstream fin("medie.in");
ofstream fout("medie.out");
int n,i,k,j,s;
float x,v[9005];
int main()
{
    fin>>n;
    for(i=1;i<=n;i++)
        fin>>v[i];
    for(i=1;i<=n;i++)
    {
        for(j=1;j<=n;j++)
        {
            for(k=j+1;k<=n;k++)
            {
                x=(v[j]+v[k])/2;
                if(i!=j and i!=k and v[i]==x)
                    s++;
            }
        }
    }
    fout<<s;
}