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.  $0,20,19,38,36,\cdots$  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? 
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