I'm happy to do the psudo-code for you as this program is really simple.Although I won't be writing the code. I suggest you write some code following the below steps and once you get stuck, just raise another quetion with the code you've achieved.
make these variables: 1-sum (perhaps global but it doesn't matter in this case) 2-min_num ( it takes in user input)
**3-max_num **(it takes in user input)
then step 4 -> do some checking to see the variables (min_num & max_num) are not empty and contain some values. You'll need to parse them as integers or whatever type you're expecting from your users such as float. It might be a good idea to display a message if users are trying to use your calculator but they aren't entering anything or entering some text which is not acceptable.
5- Use an if-statement to compare min_num < max_num
6- inside your if-statement block.. do your formula. Here where you use the sum to store the result in then return it to users. And if you want to go a little nuts with the result you can say if sum <= 0, then you can return a message to your users saying.. The total is less than or equal to zero.
For Each ctrl As Control In Me.Controls("pnlMainPanel").Controls
If ctrl.GetType Is GetType(System.Windows.Forms.Panel) Then
For Each subCtrl As Control In ctrl.Controls
If subCtrl.GetType Is GetType(System.Windows.Forms.TextBox) Then
If subCtrl.GetType Is ...