On the other hand, it will be easier, and the formula is not fun. I'll simply say, first calculate the probability of being in a group, and divide a group into two situations, which are in the same group of 1 and in the same group of 2, and the probability of the two situations is equal. They should be combined in no particular order, that is, the number above C is 2 and the number below C is 100, that is, starting from 100. . . .