This Calculator is Outdated!
Go to this link:

www.planetminecraft.com/project/redstone-calculator-20-addition-subtraction-and-display/

This calculator can do addition and subtraction. I designed it my self, and didn't use any mods building it. The answer is displayed on a redstone lamp, and also shows whether the answer is positive or negative. Download link is included. Version 2.0 will include a seven segment display, so stay tuned.

Please note: Sometimes the calculator won't work, because of a glitch that prevents sticky pistons from sticking to a block that is next to lit redstone. This can be avoided by flipping down all number entry levers before changing from addition to subtraction, or vice versa. If the calculator keeps outputting 0, make sure all number entry levers are down, and flip the addition/subtraction lever back and forth a few times. This should solve all problems. Also remember that when doing subtraction, input the first number on the right panel. (for example, for 2-5, you would input the 2 into the right panel, and the 5 into the left)
Tell me if you encounter any bugs while using the calculator. Feel free to ask me any questions!
I'm only thirteen years old, so please don't expect me to make insanely complicated redstone creations.

Additional Notes

To download:
1. Make sure Minecraft is closed.

2. Go to the Minecraft folder.
3. Click on saves.
4. Make a new folder called "Calculator"
5. Open the download link.
6. Right click on the first file/folder.
7. Click "Download linked file as..."
8. Save to the "Calculator" folder you made.
9. Repeat for all files and folders in the download link.
10. You're done! Open Minecraft and select the Calculator world.