#!/usr/bin/env python3
# %%
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
import itertools

# %%
N = int(read())

# %%
answer = 1
for n in range(2, N + 1):
    answer += 2 / n
    if n == 2:
        continue
    prods = [a * b for a, b in itertools.combinations(range(1, n), 2)]
    avg = sum(prods) / len(prods)
    answer += avg * (n - 2) / n
print(answer)