make a 4-bit adder/subtractor ( which can perform A+B and A-
B depending on value of bit say SUB..ie if SUB is 1 it will 
perform A+B ) now give A and B as inputs and make MSB of B 
as SUB bit.
suppose A=0110(6),B=1110(-2);so result will be A-B ie.,   
A+|B|=6-(-2)=8