designing a circuit that accepts n 1-bit inputs and adds them
A single bit full adder can be seen as adding three one bit numbers to produce a 2 bit number. So that takes us from 9 1 bit numbers to 3 two bit numbers. 

By carry chaining a half adder to a full adder we can produce an adder that adds two two bit numbers and produces a three bit number.

I'll leave you to figure out the final step....