import Control.Monad
import Debug.Trace

div2 :: Integer -> Integer
div2 en
    | odd en == True    = en
    | otherwise         = div2 $ div en 2

searchSosu :: Integer -> Integer -> Integer -> Integer
searchSosu pn rootn n
    | pn > rootn    = n
    | modpn == 0    = pn
    | otherwise     = searchSosu (pn + 2) rootn n
    where
        modpn = n `mod` pn

main = do
    n <- (read :: String -> Integer) <$> getLine
    let
        oddn = div2 n
        n'  | n == oddn = n
            | oddn == 1 = n
            | otherwise = oddn
        floor_rootn = floor $ sqrt $ fromInteger (n'::Integer)
        rootn_1 = floor_rootn + 1
        oddsosu = searchSosu 3 rootn_1 n'
        result  | oddsosu < 4       = oddsosu
                | (n `mod` 4) == 0  = 4
                | otherwise         = oddsosu
    print result