It adds 3 and subtracts 1 instead of adding just 1

No problem (click on my reputation if you think my help is good),

for your problem now it seems that it's a timing problem.

You can check it on the report after your compilation ended (see under the compilation report the timing section).

If you've timing problem they could depend from a lot of thing: is your clock coming from a pin?

Have you given the assignement to it in order to tell the compiler that it's a clock and that its period is 20ns?

This will really help quartus and drive its compilation.