#include <fstream>

using namespace std;

ifstream in("medie.in");
ofstream out("medie.out");

int v[9001], f[7001];

int main()
{
    int n, i, j, nr = 0;
    in >> n;
    for (i = 1; i <= n; ++i)
    {
        in >> v[i];
        ++f[v[i]];
    }
    for (i = 1; i < n; ++i)
    {
        for (j = i + 1; j <= n; ++j)
        {
            if (v[i] % 2 == v[j] % 2)
            {
                if (v[i] != v[j])
                    nr += f[(v[i] + v[j]) / 2];
                else
                    nr += max(f[v[i]] - 2, 0);
            }
        }
    }
    out << nr;
    return 0;
}