Seatpost Size on Minitrek Bicycles

If you buy a mini bike frame in Japan size 20 and without seatpost, you should watch this video until it's finished. Because I explained the size of the seatpost that fits on a Minitrek bike and isn't wrong in buying. I also explained the appropriate size of seat cleam.
Seatpost 25.4 mm Seat Cleam 28.6 mm
Seatpost 28.6 mm seat cleam 31.8 mm