/* -*- coding: utf-8 -*-
 *
 * 2070.cc:  No.2070 Icosahedron - yukicoder
 */

#include<cstdio>
#include<cmath>
#include<algorithm>
 
using namespace std;

/* constant */

/* typedef */

/* global variables */

/* subroutines */

/* main */

int main() {
  int n;
  scanf("%d", &n);

  double v = (15.0 + 5 * sqrt(5)) / 12;
  printf("%.10lf\n", v * n * n * n);
  return 0;
}