I installed ubuntu 18.04 alongside 16.04 on the same SSD. Now when I boot the machine I expected to be given the choice which ubuntu to boot, but nothing happened. Now my system tells me to press F12 to get into boot menu, but also there I don't see an option to boot 18.04 or anything like that. When I press ESC key during start up to get to GRUB menu there are also only two options: "ubuntu" and "system setup". However when I booted into 16.04 I can see my ubuntu 18.04 partition and I can access it. So how can I boot it?