Note -12 is the largest whole number in -12.1, and 13 is the largest whole number in 13.3
...- -12- -11- -10- -9- -8-...-7-8-9-10-11-12-13-...
So the numbers we have here are -12 to 13. notice that when we add the numbers up, they cancel, like -12 and 12. So, the only number we have left is $\boxed{13}$
