def main():
    N = int(input())
    if N == 1:
        return 1.0
    if N == 2:
        return 2.0
    res = 2.0
    for n in range(3, N + 1):
        res += 2 / n
        for a in range(1, n):
            for b in range(a + 1, n):
                res += 2 * a * b / (n * (n - 1))
    return res


print(main())