One way to compute the twos complement of a number is examine the number bit by bit from the LSB to the MSB$.$ In scanning from right to left, find the first bit that is $1.$ All bits to the left of this should now be complement to form the twos complement number. Design a $4-$bit twos complement generator. You should use a $4-$bit shift register $74$LS$194.$ You should use a $4-$bit counter $74$LS$19463$ too, which makes your 'Toggle' signal be asserted for a number of cycles equles to the width in bits of the register.