Oh, that's hilarious. I tested this in Python where it worked fine and forgot that C has a "broken" modulo operator:

Python:

  >>> -1 % 20
  19

C:

  printf("%d\n", (-1 % 20));
  -1

Even worse, it still fails even on unsigned:

  printf("%d\n", ((unsigned int)0 - (unsigned int)1) % 20);
  15

So you still need to add N:

  printf("%d\n", ((unsigned int)0 - (unsigned int)1 + 20) % 20);
  19

Thanks for the reminder of humility.

(Side Note: For those reading this, the C operator isn't "broken", per se. There are three properties that modulo can adhere to but two of the three are mutually exclusive.)