4bit adder-subtractor.
CI is the initial state of the carry.
A3 A2 A1 A0 Represent 1st 4bit number,
B3 B2 B1 B0 Represent 2nd 4bit number.
The SUB led represent minus sign for subtraction.
S3 S2 S1 S0 Represent the ans of 1st and 2nd 4bit number.
C Represent the carry.
