All about problems in Volume 109. If there is a thread about your problem, please use it. If not, create one with its number in the subject.
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You can solve this by exact calculation of the total perimeter of the parts of the discs that you can see from above. For each disc, see which parts of it's perimeter are covered by the discs that fall after it. Monte Carlo is probably not going to be accurate enough without getting TLE.
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