import sys
read=sys.stdin.buffer.read
readline=sys.stdin.buffer.readline
readlines=sys.stdin.buffer.readlines

n=int(readline())
ans=1
for x in range(2, n+1):
    ans+=2/x
    for a in range(1, x):
        for b in range(1, x):
            if a!=b:
                ans+=a*b/x/(x-1)
print('{:.7f}'.format(ans))