It adds 3 and subtracts 1 instead of adding just 1

Thank you Darkwave for your answer:

1. I removed 'nombre' from sensitivity process list.

2. You're right the most 5 significant bits where removed by simulator because never used. Of course, I'm using only until the bit 4. Now I've understood.

3. now with functional simulation it works!! Now the output polarity is also right. I'm testing only in the simulator. I'm using a clock period of 20ns. But I cannot understand why it is not working with timing simulation. Do you think I have to change any settings? 20ns should be enough... shouldn't?

Thank you a lot for your help!