#include <fstream>
using namespace std;
ifstream f("medie.in");
ofstream g("medie.out");

int n, a[9005], fcv[70005], sol;

int main()
{
    f>>n;
    for (int i=1; i<=n; ++i) f>>a[i], ++fcv[a[i]];

    for (int i=1; i<n; ++i)
        for (int j=i+1; j<=n; ++j)
        {
            int aux=a[i]+a[j];
            if (aux%2==0 && fcv[aux/2]) ++sol;
        }
    g<<sol<<"\n";
    return 0;
}