import typing
import sys
import math
import collections
import bisect
import itertools
import heapq
import copy

# ===TEMPLATE==
# sys.setrecursionlimit(10**7+1)
# inf = float('inf')
inf = 10**20
mod = 10 ** 9 + 7
# mod = 998244353


def ni(): return int(sys.stdin.buffer.readline())
def ns(): return map(int, sys.stdin.buffer.readline().split())
def na(): return list(map(int, sys.stdin.buffer.readline().split()))
def na1(): return list(map(lambda x: int(x)-1, sys.stdin.buffer.readline().split()))
def nall(): return list(map(int, sys.stdin.buffer.read().split()))
def flush(): return sys.stdout.flush()
def nic(): return int(sys.stdin.readline())
def nsc(): return map(int, sys.stdin.readline().split())
def nac(): return list(map(int, sys.stdin.readline().split()))
def na1c(): return list(map(lambda x: int(x)-1, sys.stdin.readline().split()))


# ===CODE===
def main():
    l = ni()
    e = l/3
    print(math.sqrt(3)/4*e*e)


if __name__ == '__main__':
    main()