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Your loop is initialized with the original length of the list, so if you change the length of your list in the meanwhile this isn't actualized (in Python) and you get an index out of range. Anyway your appoach is not good, you need to realize it. I encourage you to print stuff to the console to understand what your code does and why it fails when it does. You need to learn to debug yourself your code, that's what programming is all about.

It says consecutive numbers.