Multiply final nodes in a tree

Question

I am working with a tree, that I created by using collapsibleTree.

Each node has a character type. I would like to assign to the last nodes also a numeric value and then multiply/sum/subtract/divide them, depending on which last nodes are selected by the user.

So, for example, assuming this is the tree:

data("cities")
collapsibleTree(
  cities,
  hierarchy = c("continent", "country", "city"),
  width = 800
)

I would like to assign to each city a numeric value (e.g. population) and then I would like to have the sum of the cities that are selected by the user. For example, I would like to set Rome = 4 and San Francisco = 1, so that if both Rome and San Francisco are selected by the user, an output is returned stating 'The sum of San Francisco and Rome is 5'.

Is this something doable with the collapsibleTree interface? If not, do you have any suggestions?