static void Main(string[] args)
 {
     do
     {
         int n = SoNguyen.NhapSoNguyenDuong("Nhap n = ");
         if (TinhToan.ktdang2k(n))
         {
             Console.Write(n + " co dang 3^k");
         }
         else
         {
             Console.Write(n + " khong co dang 3^k");
         }
     } while (SoNguyen.NhapSoNguyen("\nTiep tuc (1.Tiep, 0.Khong)?") != 0);
 }