How can I use the group name of radiobuttons?

See more:

C#

WPF

group

RadioButton

Hello, I have problem with my two groups of radiobuttons(first group have 4 radiobuttons, second have 2 radiobuttons). I would like not to label any of the radiobuttons to pop up a messagebox. For example firstgroupname(level) when not marked message box show "You must marked what level do you want". I´m coding WPF Application. For clarification:
if the user does not mark in one group and in the other yes a message pops up, when the user marks in one group and in another no the other message pops up,
if he does not mark the connection of the two previous messages even in one of them. It would certainly be possible to list all the possibilities, but there would be a lot of it, so I ask about the possibilities of the group

What I have tried:

For example:

if (plus.IsChecked.HasValue == true && plus.IsChecked.Value == true && low.IsChecked.HasValue == true && low.IsChecked.Value == true)

this is how I get it when it is chosen from both groups

else if (level.IsChecked.HasValue == false && level.IsChecked.Value == false && difficul.IsChecked.HasValue == true && difficul.IsChecked.Value == true)
           {
               MessageBox.Show("You must marked what level do you want!");

           }

like this
Is it possible?

Thank you for any advice!!!