hello everyone

I want to do the sum ��and subtraction of values ��as design in xilinx ISE i had in input values ��for example 2 3 5 4 1 i must do the sum then substruct for exemple
2+3-5+4-1
I have two ideas, but with problems
1- I use ACC8 which is the sum or subtraction if add = 1 -> sum
add = 0 -> subtraction, but I do not understand how it works at the phase sousraction here is an example in the attachments
2-I made ��a block that is a compliment to 2 if the number is signed but how can i do th test
if you have any idea I really need
thank you in advance