Concatenating literal, with ActiveCell and FormulaR1C1

I can't find a way to do this. If I don't use the literal "R", it works fine, but I need that R. Any ideas? Here's my line:

ActiveCell.FormulaR1C1 = "=R[-2]C / R[-1]C"

I want to concatenate an "R" at the end. How to do it?

If I just try to add

& "R"

I get "Application-defined or object-defined error. "