import sys

sys.setrecursionlimit(10 ** 6)
int1 = lambda x: int(x) - 1
p2D = lambda x: print(*x, sep="\n")
def II(): return int(sys.stdin.readline())
def MI(): return map(int, sys.stdin.readline().split())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def SI(): return sys.stdin.readline()[:-1]

def main():
    n=II()
    ans=1
    s=0
    for i in range(2,n+1):
        s+=(i-1)*(i-1)*(i-2)//2
        ans+=2/i+2*s/i/(i-1)
    print(ans)

main()