#include <fstream>

using namespace std;
int v[50000], f[50000], n, i, j, sol;
int main () {
    ifstream fin("medie.in");
    ofstream fout("medie.out");
    fin>>n;
    for (int i=1;i<=n;i++) {
        fin>>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) {
                sol += f[ (v[i]+v[j])/2 ];
                if (v[i] == v[j])
                    sol -= 2;
            }
        }
    fout<<sol;

    return 0;
}