Design a 4-bit circuit for 2s-complement, base on the formula below, using only inverters, 2 input AND gates and 2 input OR gates. Design your circuit to be as fast as possible. What is the worst-case propagation delay for this circuit, assuming a 1 ns delay for each inverter and a 2 ns delay for each AND gate and OR gate? (hint: for all i>0 use the formula below) , what about i=0? Note that x is input and x3 as the most significant bit and is y is the output, similarly y3 is the most significant bit.