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