## https://yukicoder.me/problems/no/2971

import math
        

def main():
    N = int(input())

    n = 10 ** 6

    answer = 0
    for m in range(n):
        x = (m ** 3) / n +  (n ** 2) * (N ** 3)
        y = math.sqrt(x)
        answer += y
    answer /= (n ** 2)

    answer *= 10 ** 6
    answer = int(answer)
    answer = str(answer)
    answer = "0000000" + answer

    answer = str(int(answer[:-6])) + "." + answer[-6:]
    print(answer)











if __name__ == "__main__":
    main()