(Note too the problem asks for the general solution, too, with a possibly weighted coin)

Let me try to recall how the heck this problem works. The vague idea, which the next post is a clue, is to express the likelihood of success from a given position in terms of the success from neighboring positions– it turns out it’s a special weighted average. But missing all the details… (There is some cleverness needed to set it up right in the first place)

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