Adding Subtotals in Microsoft Access Reporting - Microsoft Community

i have report grouped on year , month, subtotals @ end of 2016 data , subtotals @ end of 2017 data.  i'm trying subtract 2017 subtotals 2016 subtotal , can't figure out how it.  below quick example.  i'm looking how subtract 30 12.

                              widgets

jan-16                         5

feb-16                         4

mar-16                        3

ytd mar-16                 12

jan-17                          10

feb-17                          10

mar-17                          10

ytd mar-17                  30

Office / Access / Other/unknown / Office 2010