def f(n):
    return ((n*(n+1)//2)**2 - n*(n+1)*(2*n+1)/6)/(n*(n-1))

n = int(input())
ans = 2.0
for i in range(2,n):
    ans += 1*2/(i+1)
    ans += f(i)*(i-1)/(i+1)
print(ans)