#include <stdio.h>
#define EPS 1e-9
#define TRUE (1<2)
#define FALSE (!TRUE)
// The function that checks if two values are equal
int eq (double a, double b);
// The function that searchs from the lo to the hi for the max area
// The function return the max area found
double search(double lo, double hi, double length);
// The function that returns the area (could be repurposed for
// Moore)
double foo(double total, double upper);
// The main function
int main(){
    // Read in the length
    double length;
    scanf("%lf", &length);
    // Initialize the lo and hi bound of the top rod of the  frame
    double lo = 0;
    double hi = 1;
    // Check if we can use a better upper bound for the length of the top rod
    while (foo(length, hi) < foo(length, hi * 2))
        hi = hi * 2;
        // Double one last time to ensure we have the maximum within our range
        hi = hi * 2;
        // Compute the answer by searching from the lo to the hi bound
        double ans = search(lo, hi, length);
        // Print the answer
        printf("%lf\n", ans);
        // Return the answer
        return 0;

}
double search(double lo, double hi, double length)
{
    // Check if the lo and hi value are approximately the same
    if (eq(lo, hi))
        return foo(length, lo); // Get the area of the frame
        // Compute the mid points using the 2 thirds trick discussed in class
    // Note this breaks the search space into 3 "equal" parts
    double m1 = (2 * lo + hi) / 3;
    double m2 = (lo + 2 * hi) / 3;
    // Compute the area using the first and second mid points respectively
    double area1 = foo(length, m1);
    double area2 = foo(length, m2);
    // Check if area 1 is worse than area 2
    if (area1 < area2) {
        // Since area 1 is worse eliminate the first section of the search move up the lower bound  lo = m1;
        return search(m1, hi, length);

    }
    // Area 1 is better move down the hi bound
    return search(lo, m2, length);
}

    int eq (double a, double b)
    {    // Get the difference
    double diff = a - b;
    if (diff < 0)
        diff = -diff; // absolute value of the difference
    if (diff < EPS)
        return TRUE; // ABSOLUTE ERROR CHECK PASSED
    if (a < 0)
        a = -a; // abs
    if (b < 0)
        b = -b; // abs
    if (a < b)
        a = b; // max
        // Scale the error by the magnitude of the big value
        // This will give a new relative error to check against
    if (a * EPS > diff)
        return TRUE; // RELATIVE ERROR CHECK PASSED

        return FALSE; // Not equal values
        }

        // This function computes the area of the frame given
        // 1. The total length of the wooden rod
        //2. The width of the frame (upper wood piece)
        double foo(double total, double upper){
            // Compute the remaining length after using the bottom and top
            double remainingRod = total - 2 * upper;
            // Compute the height by using the remaining rod in 6 different pieces
            double small = remainingRod / 6;
    // Return the area (product of the height and the width)
    return small * upper;
}