import Text.Printf

getInts :: IO [Int]
getInts = map read . words <$> getLine

main :: IO()
main = do
  _ <- getLine
  a <- getInts
  printf "%f" $ slv a 

slv :: [Int] -> Double
slv a = 1.0 / val
  where val = sum $ map (\x -> 1.0 / (fromIntegral x)) a