N = int(input())
ans = 2.0
for i in range(3,N+1):
    ans += (1 / i) * 2
    for l in range(1,i):
        for r in range(1,i):
            if l == r:
                continue
            ans += l * r * (1 / (i-1)) * (1 / i)
print(ans)