how do i get the first bit of a 16bit number using a logic gate simulator?

Question

I am currently building an Alu computer component in Logical Circuit simulator. I want to check if the result i get from the Alu (a 16 bit number) is positive or negative, I know i need to use the two's complement (in whice the most significant bit tells us if the number is positive or negative). I tried to take the answer (Output from the mux16) and run it through a 16 bit splitter and then run it through a not gate(since i want my ng output[1bit] to be equal to 1 if the number is negative) and take the upper connection of the splitter since i assumed it would be the first bit but it seems to not work this way. Is the problem in the implemenation?