-- | Generates the sequence of prime numbers.
-- Based on <http://qiita.com/little_Haskeller/items/614a3ae20a517c19bb1f this article> .
primes :: Integral a => [a]
primes = map fromIntegral primes'
    where
        primes' = [2, 3, 5] ++ sieve2 5 7 (drop 2 primes')
        sieve2 m s (p : ps) = [n | n <- ns, gcd m n == 1] ++ sieve2 (m * p) (p * p) ps
            where ns = [x + y | x <- [s, s + 6 .. p * p - 2], y <- [0, 4]]

-- | Factorizes the given integer using 'primes'.
-- Based on <https://wiki.haskell.org/Testing_primality this page>.
factorize :: Integral a => a -> [a]
factorize 1 = []
factorize n = factorize' n primes
    where
        factorize' n ps@(p : pr)
            | p * p > n    = [n]
            | m == 0       = p : factorize' d ps
            | otherwise    = factorize' n pr
            where (d, m) = divMod n p

getMinimum :: Integral a => a -> a
getMinimum n
    | h > 2        = h
    | elem 3 fs    = 3
    | notElem 2 fr = head fr
    | otherwise    = 4
    where
        fs@(h : fr) = factorize n

main :: IO ()
main = print . getMinimum =<< readLn