The problem says:

A combinational circuit has as inputs two numbers (A, B) of four bits each, which must be natural BCD (from 0 to 9), two operating mode inputs (x, y) and the output is displayed on two 7-segment displays. If some of the numbers is not BCD, regardless of the inputs (x, y) a zero should be displayed. If the inputs are BCD, it must respond to the following operation table:

**X Y Function**

0 1 B

1 0 A

1 1 A+B

0 1 B

1 0 A

1 1 A+B

can someone give me a help to start it?

Thanks!