You need m-1 full adders.
Except the first bit rest all m-1 bits require the full adder.
Let, A= a1 a2 a3 a4
B= b1 b2 b3 b4
For 1st bit we need only input a1 and b1 no need of carry so we can satisy by using half adder which req 2 i/p.
2nd bit for a2 + b2 we need to add carry generated for 1st bit. So here we have 3 i/p, i.e a2+b2+c1
So we use FULL ADDER from b2 to b4 (3 full adder for 4 bit input)
So, overall we can manage addition with M-1 full adders..