This is my design for a 2-digit adder-subtractor BCD circuit. The design performs well with addition, but for subtraction, it calculates 33 - 73 = 60, which is incorrect. Can anyone help me figure out the issue?

I’ve used a 74LS83 IC along with a bunch of XOR gates, AND gates, NOT gates, and a 7-segment display to show the output. The design was implemented using Proteus 8 Professional.

 

I think 60 is the correct answer.

33 - 73 = 33 + (100 - 73) = 33 + 27 = 60

Another way of looking at it is:
33 - 73 = -40 = (100 - 40) = 60