下面程序的输出为（    ）。

#include <iostream>

using namespace std;

int main() {

      int N = 15, cnt = 0;

      for (int x = 0; x + x + x <= N; x++)

            for (int y = x; x + y + y <= N; y++)

                  for (int z = y; x + y + z <= N; z++)

                        cnt++;

      cout << cnt << endl;

      return 0;

}