CF-gym100338-A Shitália

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博客介绍了一种特殊的星形岛屿——Shitalia，该岛屿由外圈半径为R和内圈半径为r的点构成。文章提供了计算这种星形岛屿面积的方法，并通过几个样例展示了如何使用特定的公式来计算不同配置下岛屿的面积。

题目点我！

standard input/output

After suddenly becoming a billionaire, Shi adopted YOLO as his motto and decided to buy a small european island, which he named Shitalia. Many people decided to move to Shitalia to run away from the economic crisis, and then created the Shitalian mafia, led by Dom Shi.

The Shitalian economy is based on smuggling spoiled cheese, but, as you can imagine, there aren't many people interested in spoiled cheese. Therefore, Shi decided to earn money with land rental.

Shitalia is a star-shaped island. Shi is very picky, so he chose a Shi-regular n-star. An n-star is Shi-regular if there are n outer points andn inner points alternated. The outer points must lay on a circle with radius R and the inner points must lay on a circle with radius r and all the points must be equally spaced.

Print the area of the Shitalia.

$\text{[math]}$ Shi-regular 6-star and 4-star.

Input

The first line contains three integers 3 ≤ n ≤ 20, 1 ≤ R ≤ 10⁶ and 1 ≤ r ≤ R, indicating the number of points of Shitalia, the radius of the outer circle and the radius of the inner circle.

Output

Print a single real number - the answer to the problem with absolute or relative error of at most 10^- 6.

Sample Input

Input