The ripple carry adder below adds two 16-bit numbers X and Y (i.e. S=X+Y), where X=X₁₅X₁₄X₁X₀ and Y=Y₁₅Y₁₄Y₁Y₀. Assume we are using twos complement representation for our signed numbers, in which flipping all the bits of a number Y and adding one to it will give -Y.

Modify the circuit below by including an input line P that specifies whether the circuit will add (that is, it will perform X+Y when P = 0) or subtract (that is, it will perform X-Y when P = 1). Hint: X-Y = X+(-Y) and also note that Q XOR 1 flips Q (that is Q XOR 1 = Q').

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