I need help, because I forgot what to do to change this back.

I opened a spreadsheet this morning and in one of the cells, instead of the
regular formula I was expecting, (eg, sum e30:e50), you get something like
this:

SUM(R[-27]C:R[-3]C)

I am sure this is an easy re-set to the normal mode.

Thanks for any help you can provide,
Ann