Dashes are not legal in R object or column names. To avoid an error, you need to enclose the illegal name in backticks. Your code will work if you do the following:

Quotation marks will work for indexing inside braces, so only the reference to the object needs to be enclosed in backticks. However, to use the dollar-sign notation, you'd need backticks for the column name too, giving this abominable bit of code:

It's normally better to work with legal column names and only change to "real-world" names when outputting to graphs, tables, etc.

As a new R user, the most intuitive and transparent approach for changing object and column names is probably to just rename the data explicitly for each symbol. We'll switch from hyphens to underscores, since underscores are legal:

A more efficient, but less intuitive and transparent approach is to put the data in a list and operate on the list: