The way I thought about it, you have to roll around the larger one and you have to roll around yourself (+1). So if they are two same size circles, it's 1 for the other and 1 for yourself. If the other one has 3 times bigger radius, it is 3 for the bigger one and 1 for yourself. Etc.
Not sure if it is mathematically accurate but it seemed to make sense to me.
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