// ### task CIRCLE AREA ###

/* ### Description ###

There is a length of a circle. Find the area of the circle.

Input:
A positive real number, the length of the circle.

Output:
A real number, the area of the circle.

*/

#include <iostream>

#include <iomanip>

#define _USE_MATH_DEFINES
#include <cmath>

#ifndef M_PI
    #define M_PI 3.14159265358979323846
#endif

using namespace std;

int main()
{
	//lenght of circle
	double l;
	
	//reading length
	cin >> l;
	
	//area of circle
	double s;
	
	/*
		know:
			l = 2 * pi * r;
			
			s = pi * r^2;
		
		l^2 = 4 * pi^2 * r^2;
		
		l^2 / (4 * pi) = pi * r^2;
		
		l^2 / (4 * pi) = s;
		
		so: 
			s = l^2 / (4 * pi);
	*/
	
	//calculate the area
	s = pow(l, 2) / (4 * M_PI);
	
	//printing area of circle
	cout << fixed << setprecision(9) << s;
	
	return 0;
}