CodeForces 1100C NN and the Optical Illusion 数学

http://codeforces.com/problemset/problem/1100/C

NN is an experienced internet user and that means he spends a lot of time on the social media. Once he found the following image on the Net, which asked him to compare the sizes of inner circles:

It turned out that the circles are equal. NN was very surprised by this fact, so he decided to create a similar picture himself.

He managed to calculate the number of outer circles nn and the radius of the inner circle rr. NN thinks that, using this information, you can exactly determine the radius of the outer circles RR so that the inner circle touches all of the outer ones externally and each pair of neighboring outer circles also touches each other. While NN tried very hard to guess the required radius, he didn't manage to do t