Conceptual model with a constraint

I'm trying to make a conceptual model of this use case:

The Actor can modify the budget but after the modification the sum of all the budget elements percentages must be 100% or 0% if there are no elements in the budget.

How can I include this constraint in my model?
The model can also be found here

You should just put a constraint on the contains property. In MagicDraw one can show the label for a constraint or the actual constraint in OCL.

Here is an example with the label only, which is probably more appropriate for the audience of a conceptual model:

Here is an example with OCL:

A little less readable, eh? 😉