I would like to know if its Possible to implement the attached question using Half Adders only .
I've implemented it using half adders and some external gates.
is it possible to design this using only half adders.

P.S A clue or hint towards the approach is fine.

 

Could you implement the circuit if all you had were a bunch of 2-input NAND gates?

If so, can you implement a 2-input NAND gate using nothing but half-adders?