import Control.Applicative
import Control.Monad
import Control.Monad.ST
import Data.Vector.Unboxed.Mutable (STVector)
import qualified Data.Vector.Unboxed.Mutable as VM

main :: IO ()
main = solve <$> readLn >>= print

solve :: Int -> Int
solve n = runST $ do
  sv <- sieve 100003
  loop sv n
  where
    loop sv i = do
      b <- VM.read sv i
      if b then return i
        else loop sv (i + 1)

sieve :: Int -> ST s (STVector s Bool)
sieve n = do
  sv <- VM.replicate (n + 1) True
  VM.write sv 0 False
  VM.write sv 1 False
  forM_ ixs $ \i -> do
    b <- VM.read sv i
    when b $ forM_ [i * i, i * i + i .. n] $ \j -> VM.write sv j False
  return sv
  where
    ixs = takeWhile ((<= n) . (^2)) [2..]
