#include <fstream>

using namespace std;

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

int n;
int v[9001], frecv[7001], ans;

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