#pragma region template
// clang-format off

#ifdef LOCAL_NDEBUG
#include <headers.hpp>
#endif

// #define USE_EXTERNAL_CONTAINERS
// #define NO_PRINT_INF

#ifdef USE_EXTERNAL_CONTAINERS
#define PROPER
#include <boost/container/flat_map.hpp>
#include <boost/container/flat_set.hpp>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
template <class T>          using rb_tree    = __gnu_pbds::tree<T, __gnu_pbds::null_type, std::less<T>, __gnu_pbds::rb_tree_tag, __gnu_pbds::tree_order_statistics_node_update>;
template <class S, class T> using hash_table = __gnu_pbds::gp_hash_table<S, T>;
#endif

#ifdef LOCAL_DEBUG
#if (defined USE_EXTERNAL_CONTAINERS) || (defined NO_PRINT_INF) || (defined PROPER)
#include <../src/debugger.hpp>
#else
#include <debugger.hpp>
#endif
#endif

// #define PROPER

#if (defined __INTELLISENSE__) && (!defined PROPER)
#define NDEBUG
namespace std {
#endif

#include <cassert>
#include <cctype>
#include <cmath>
#include <cstddef>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <algorithm>
#include <array>
#include <bitset>
#include <complex>
#include <deque>
#include <functional>
#include <iostream>
#include <iomanip>
#include <iterator>
#include <limits>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <regex>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <string_view>
#include <tuple>
#include <typeinfo>
#include <type_traits>
#include <utility>
#include <vector>

#if (defined __INTELLISENSE__) && (!defined PROPER)
  using namespace std;
}
#endif

#ifdef LOCAL_DEBUG
#define STR(x) #x
#define STRINGIFY(x) STR(x)
#define FILE_LINE "[Debug] ./" __FILE__ ":" STRINGIFY(__LINE__)
#define see(...) debugger::multi_print(#__VA_ARGS__, __VA_ARGS__)
#define see2(arg) arg.debug_print(#arg)
#define here(...) debugger::os << FILE_LINE << " in " << __func__ << ": \e[32mReached\e[39m\n"
#define com(msg) debugger::os << FILE_LINE << " in " << __func__ << ":\n    \e[36mComment:\e[39m " << msg << "\n"
#define err(msg) debugger::os << FILE_LINE << " in " << __func__ << ":\n    \e[31mError:\e[39m " << msg << "\n"
#define local(...) do { __VA_ARGS__ } while (0)
#define alter(x, y) x
#else
#define see(...) (static_cast<void>(0))
#define see2(arg) (static_cast<void>(0))
#define here(...) (static_cast<void>(0))
#define com(msg) (static_cast<void>(0))
#define err(msg) (static_cast<void>(0))
#define local(...) (static_cast<void>(0))
#define alter(x, y) y
#endif

#if (defined LOCAL_DEBUG) && (!defined NOWARN)
#define warn(msg) debugger::os << FILE_LINE << " in " << __func__ << ":\n    \e[33mWarning:\e[39m " << msg << "\n"
#else
#define warn(msg) (static_cast<void>(0))
#endif

#if (defined LOCAL_DEBUG) || (defined LOCAL_NDEBUG) || (defined __INTELLISENSE__)
#define NOEXCEPT
#define M_assert(expr) assert(expr)
#define O_assert(expr) assert(expr)
#else
#define NOEXCEPT noexcept
#define M_assert(expr) do {if(__builtin_expect(!(expr), 0)) {auto p = static_cast<std::int64_t*>(std::malloc(1 << 30)); for (int i = 0; i < (1 << 27); p[i] = 1, i += (1 << 9)); std::cerr << (*p);}} while (0)
#define O_assert(expr) do {if(__builtin_expect(!(expr), 0)) {const auto X = std::string(1000, '-'); for(int i = 0; i < (1 << 18); i++) std::cout << X;}} while (0)
#endif

#define as(type, val) static_cast<std::decay_t<type>>(val)
#define INDIRECT(...) __VA_ARGS__

#define rep(loop_var, loop_end) \
  for ( \
    auto loop_var = as(std::make_signed_t<decltype(loop_end)>, 0); \
    loop_var < as(decltype(loop_var), loop_end); \
    ++loop_var \
  )
#define rng(loop_var, loop_start, loop_end, loop_increment) \
  for ( \
    auto loop_var = as(INDIRECT(std::make_signed_t<std::common_type_t<decltype(loop_start), decltype(loop_end)>>), loop_start); \
    ((loop_increment) > 0) ? (loop_var < as(decltype(loop_var), loop_end)) : (loop_var > as(decltype(loop_var), loop_end)); \
    loop_var += (loop_increment) \
  )
#define erng(loop_var, loop_start, loop_end, loop_increment) \
  for ( \
    auto loop_var = as(INDIRECT(std::make_signed_t<std::common_type_t<decltype(loop_start), decltype(loop_end)>>), loop_start); \
    ((loop_increment) > 0) ? (loop_var <= as(decltype(loop_var), loop_end)) : (loop_var >= as(decltype(loop_var), loop_end)); \
    loop_var += (loop_increment) \
  )

[[maybe_unused]] constexpr int         INF   = 1000000005;
[[maybe_unused]] constexpr long long   LINF  = 1000000000000000005LL;
[[maybe_unused]] constexpr double      EPS   = 1e-9;
[[maybe_unused]] constexpr long double LEPS  = 1e-14L;
[[maybe_unused]] constexpr int         dy[9] = {1, 0, -1, 0, 1, 1, -1, -1, 0};
[[maybe_unused]] constexpr int         dx[9] = {0, 1, 0, -1, -1, 1, 1, -1, 0};

template <class... Ts> constexpr auto Min(const Ts... args) { return std::min({ as(std::common_type_t<Ts...>, args) ... }); }
template <class... Ts> constexpr auto Max(const Ts... args) { return std::max({ as(std::common_type_t<Ts...>, args) ... }); }

// clang-format on
#pragma endregion

void solve() {
  long double L;
  std::cin >> L;
  std::cout << std::fixed << std::setprecision(15) << (L / 6) * (L * std::sqrt(3) / 6) << "\n";
}

int main() {
  std::ios_base::sync_with_stdio(false);
  std::cin.tie(nullptr);
  solve();
}