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In challenge 5 you are given 20 tokens to start with and you will beat the level if you somehow manage to get your hands on any additional tokens. To win this challenge we need to trigger an overflow or underflow. The overflow is a situation when an integer (signed integer) reaches its byte size. The next element added will return the first variable element. If you subtract 1 from a variable that is 0, it will change the value to 255. There are two options to use Openppelin to use all the mathematical operators.