import sys
readline=sys.stdin.readline

N=int(readline())
ans=1
for i in range(2,N+1):
    s1=i*(i-1)//2
    s2=i*(i-1)*(2*i-1)//6
    ans+=2/i
    if i>=3:
        ans+=(s1*s1-s2)*(i-2)/(i-1)/(i-2)/i
print(ans)