The progress I made is that I used a XOR gate with the complement of R, to be fed as input to each of the first 4 inputs of the decrementer. It doesn't seem to work for all input combinations. I would appreciate some suggestions on modification, as this doesn't lead to correct outputs. I tried to find out the logical expressions of the incrementer and decrementer individually to link them together using the control input R. Please have a look at the attached figure.
Kindly refer to the attached figure. The expressions that the output needs to assume in each case (R=0 and R=1) have been derived already by me. I can't find a way to link them together.