Optional:
Warren and Naida make a list of numbers. They start with 0, then
Warren adds 20, then Naida subtracts 1, then Warren adds 19, then
Naida subtracts 2.
0, 20, 19, 38, 36,...
This pattern continues until Warren adds 1 and Naida subtracts 20,
which ends up back at 0. What is the largest number in their list?