In demultiplexer, inputs is inserted serially and then it gives multiple outputs which are in parallel form.
Digital Logic MCQ - Combinational Circuits
We need an adder for every bit. So we should need m full adders. A full adder adds a carry bit to two inputs and produces an output and a carry.
But the most significant bits can use a half adder, which differs from the full adder as in that it has no carry input, so we need m-1 full adders in m bit parallel adder.
