#include <algorithm>
#include <fstream>
using namespace std;
int v[9005], a[9005];
int main()
{   ifstream f("medie.in");
    ofstream g("medie.out");

    int n,cont=0;
    f>>n;
    for (int i=1;i<=n;++i)
        f>>v[i];
    for (int i=1;i<=n;++i)
        for (int j=i+1;j<=n;++j)
           if ((v[i]+v[j])%2==0)
            {   a[(v[i]+v[j])/2]++;
                if ((v[i]+v[j])/2==v[i]) cont--;
                if ((v[i]+v[j])/2==v[j]) cont--;
            }

    for (int i=1;i<=n;++i)
        cont=cont+a[v[i]];
    g<<cont;

    return 0;

}