I would like to ask a question about half adder/half subtractor. I have a,b inputs and x control input. Outputs are sum and carry. I know how to draw half adder/half subtractor circuit diagram. However, there is a question: " Design an one bit half adder/subtractor circuit. There are a,b inputs and also x control input. Sum(S) and Carry(C) are outputs. I need to ADD operation for x=1 and SUBTRACTION operation for x=0.

Please, could you tell me how I can create the circuit. I have not understood that if I have 3 inputs then can I use half adder ? (If I create logic table, this looks like full adder circuit.)

Thanks for any help.