Hello, I am a student and need help creating a 4-bit adder/subtractor on logisim which displays the result in a 7-Segment display. My current circuit adds my inputs up to "9" and subtracts up to "0" which is displayed on a single 7-segment. although, I also need to represent negative results with two displays (one being the negative sign and the other being the digit). I also need to display an "E" when overflow occurs. Attached is my current circuit ( the other variation uses only one chip for the adder/subtractor). I greatly appreciate any help 
