import Data.List
import Data.Char
import Data.Maybe
import qualified Data.ByteString.Char8 as BS8

main :: IO ()
main = do
    n <- fst . fromJust . BS8.readInteger <$> BS8.getLine
    i <- unfoldr (BS8.readInteger . BS8.dropWhile isSpace) <$> BS8.getLine
    let a = [1 / (toRational x) | x <- i]

    print $ fromRational (1 / (foldr (+) 0 a))
