CIRCLE AREA

// ### 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;
}