import Data.List
import Control.Monad
import qualified Data.Set as S

primes :: [Int]
primes = 2 : 3 : [x | i<-[1..], j<-[-1,1], let x = 6*i+j, isPrime x]
 where isPrime n = null [i | i <- takeWhile (\x -> x^2 <= n) primes, mod n i == 0]

main = do
 getLine
 as <- map read . words <$> getLine
 putStrLn $ maybe "-1" show (rprime as)

rprime :: [Int] -> Maybe Int
rprime as = guard ((not.null) ds) >> maximum ds
 where
  sas = S.fromList (concatMap show as)
  ps = takeWhile (<= 5000000) primes
  sps = S.fromList ps
  ips = zip [0..] ps
  pcands = filter (S.null . flip S.difference sas . S.fromList . show . snd) ips
  punits = filter ((== sas) . S.fromList . concat . map show) (groupBySeq pcands)
  ds = map (diffs sps) punits

groupBySeq :: [(Int, Int)] -> [[Int]]
groupBySeq [] = []
groupBySeq xs@((i,p):ips) = g : groupBySeq (drop (length g) xs)
 where g = map (snd.snd) $ takeWhile (\(i,(j,_)) -> i==j) (zip [i..] xs)

diffs :: S.Set Int -> [Int] -> Maybe Int
diffs sps ps = do
  plb <- (succ <$> S.lookupLT (minimum ps) sps) `mplus` (Just 1)
  phb <- (pred <$> S.lookupGT (maximum ps) sps) `mplus` (Just 5000000)
  return (phb - plb)