#!/usr/bin/env python3

def main():

    L = int(eval(input()))



    ans = 0

    for i in range(min(L, 1000)):

        for j in range(1000):

            a = i + (j / 1000)

            b = (L - a) / 2

            ans = max(ans, a * b * b)

    print(ans)





if __name__ == '__main__':

    main()

#!/usr/bin/env python3

def main():

    L = int(eval(input()))



    print((L ** 3 / 27))





if __name__ == '__main__':

    main()

#!/usr/bin/env python3
def main():
    L = int(eval(input()))
    ans = 0
    for i in range(min(L, 1000)):
        for j in range(1000):
            a = i + (j / 1000)
            b = (L - a) / 2
            ans = max(ans, a * b * b)
    print(ans)


if __name__ == "__main__":
    main()

#!/usr/bin/env python3
def main():
    L = int(eval(input()))
    print((L**3 / 27))


if __name__ == "__main__":
    main()

-    ans = 0 -    for i in range(min(L, 1000)): -        for j in range(1000): -            a = i + (j / 1000) -            b = (L - a) / 2 -            ans = max(ans, a * b * b) -    print(ans) +    print((L**3 / 27))