import           Control.Monad
import           Control.Monad.ST
import           Data.Bits
import qualified Data.Vector                 as V
import qualified Data.Vector.Mutable         as VM
import qualified Data.Vector.Unboxed         as VU
import qualified Data.Vector.Unboxed.Mutable as VUM

main :: IO ()
main = readLn >>= zeta >>= print

zeta :: Int -> IO Double
zeta n = do
    let primes = getPrimeVector 10000
    xm <- V.unsafeThaw $ V.fromList [1.0]
    forM_ [0 .. (VU.length primes - 1)] $ \i -> do
        let p = fromIntegral $ primes VU.! i
            x = 1.0 / (1.0 - p ** (- (fromIntegral n)))
        VM.modify xm (\a -> a * x) 0
    (V.! 0) <$> V.unsafeFreeze xm

getPrimeVector :: Int -> VU.Vector Int
getPrimeVector top = VU.filter (/= -1) . VU.imap (\i check -> if i == 0 then 2 else if check then i * 2 + 1 else -1) $! runST $ do
    let m = (top - 1) `shiftR` 1
        r = floor . sqrt . fromIntegral $ (top + 1)
    sieve <- VU.unsafeThaw $ VU.replicate (m + 1) True
    forM_ [1 .. r `unsafeShiftR` 1] $ \i -> do
        isPrime <- VUM.unsafeRead sieve i
        when isPrime $ do
            forM_ [2 * i * (i + 1), 2 * i * (i + 2) + 1 .. m] $ \j -> do
                VUM.unsafeWrite sieve j False
    VU.unsafeFreeze sieve