/* a program to compute the area of a pentagon given the distance between the center and one of the vertices */

import java.util.*;			// for a Scanner

public class Prog4_1
{
	public static void main(String[] args)
	{
		double r, s, area;								// r = radius, s = side
		Scanner in=new Scanner(System.in);				// create a Scanner
		System.out.print("Enter the distance between the central point of the pentagon and a vertex (r):  ");
		r = in.nextDouble();							// radius should be a double
		s = 2 * r * Math.sin(Math.PI/5);				// compute the side
		area = 5 * s * s / (4 * Math.tan(Math.PI/5));	// compute the area
		area = Math.round(area * 100) / 100.0;			// reduce the number of decimal points of accuracy to 2
		System.out.println("The area of the pentagon is " + area);	// output result
	}
}