-- https://github.com/minoki/my-atcoder-solutions
{-# LANGUAGE BangPatterns #-}
import Data.Char (isSpace)
import Data.List (unfoldr)
import Control.Monad
import qualified Data.Vector.Unboxed as U
import qualified Data.Vector.Unboxed.Mutable as UM
import qualified Data.ByteString.Char8 as BS
import qualified Data.IntMap.Strict as IntMap
import qualified Data.IntSet as IntSet
import Data.Foldable
import Control.Monad.ST

main = do
  [n,m] <- unfoldr (BS.readInt . BS.dropWhile isSpace) <$> BS.getLine
  edges <- U.replicateM m $ do
    [u,v] <- unfoldr (BS.readInt . BS.dropWhile isSpace) <$> BS.getLine
    return (u-1,v-1)
  [s,t] <- map (subtract 1) . unfoldr (BS.readInt . BS.dropWhile isSpace) <$> BS.getLine
  let graph :: IntMap.IntMap IntSet.IntSet
      graph = U.foldl' (\m (u,v) -> insertGraph (3 * u) (3 * v + 1)
                                    $ insertGraph (3 * u + 1) (3 * v + 2)
                                    $ insertGraph (3 * u + 2) (3 * v)
                                    $ m) IntMap.empty edges
      result = runST $ do
        visited <- UM.replicate (3 * n) False
        let !target = 3 * t
        -- bfs :: Int -> [Int] -> ST s (Maybe Int)
        let bfs !depth ss = do
              forM_ ss $ \u ->
                UM.write visited u True
              let ts = foldMap (\u -> IntMap.findWithDefault IntSet.empty u graph) ss
              if target `IntSet.member` ts
                then return $ Just (depth `quot` 3)
                else do ts' <- filterM (\v -> not <$> UM.read visited v) (IntSet.toList ts)
                        if null ts'
                          then return Nothing
                          else bfs (depth+1) ts'
        bfs 1 [3 * s]
  case result of
    Nothing -> putStrLn "-1"
    Just d -> print d

-- insertGraph key val == IntMap.insertWith IntSet.union key (IntSet.singleton val)
insertGraph :: Int -> Int -> IntMap.IntMap IntSet.IntSet -> IntMap.IntMap IntSet.IntSet
insertGraph !key !val = IntMap.alter (Just . f) key
  where f Nothing = IntSet.singleton val
        f (Just set) = IntSet.insert val set

foldMap_IntSet :: (Monoid n) => (Int -> n) -> IntSet.IntSet -> n
foldMap_IntSet f set = go set
  where
    go set = case IntSet.splitRoot set of
               [] -> mempty
               [x] -> foldMap f (IntSet.toList x)
               xs -> foldMap go xs

forM_IntSet :: Monad m => IntSet.IntSet -> (Int -> m ()) -> m ()
forM_IntSet set f = go set
  where
    go set = case IntSet.splitRoot set of
               [] -> return ()
               [x] -> forM_ (IntSet.toList x) f
               xs -> forM_ xs go

foldMapM_IntSet :: (Monoid n, Monad m) => (Int -> m n) -> IntSet.IntSet -> m n
foldMapM_IntSet f set = go set
  where
    go set = case IntSet.splitRoot set of
               [] -> return mempty
               [x] -> foldlM (\x v -> mappend x <$> f v) mempty (IntSet.toList x)
               xs -> foldlM (\x set' -> mappend x <$> go set') mempty xs
