Problem:
Score : 500 points

Problem Statement

Ken loves ken-ken-pa (Japanese version of hopscotch). Today, he will play it on a directed graph G.
G consists of N vertices numbered 1 to N, and M edges. The i-th edge points from Vertex u_i to Vertex v_i.

First, Ken stands on Vertex S. He wants to reach Vertex T by repeating ken-ken-pa. In one ken-ken-pa, he does the following exactly three times: follow an edge pointing from the vertex on which he is standing.

Determine if he can reach Vertex T by repeating ken-ken-pa. If the answer is yes, find the minimum number of ken-ken-pa needed to reach Vertex T. Note that visiting Vertex T in the middle of a ken-ken-pa does not count as reaching Vertex T by repeating ken-ken-pa.

Constraints

2 \leq N \leq 10^5

0 \leq M \leq \min(10^5, N (N-1))

1 \leq u_i, v_i \leq N(1 \leq i \leq M)

u_i \neq v_i (1 \leq i \leq M)

If i \neq j, (u_i, v_i) \neq (u_j, v_j).

1 \leq S, T \leq N

S \neq T

Input

Input is given from Standard Input in the following format:

N M
u_1 v_1
:
u_M v_M
S T

Output

If Ken cannot reach Vertex T from Vertex S by repeating ken-ken-pa, print -1.
If he can, print the minimum number of ken-ken-pa needed to reach vertex T.

Sample Input 1

4 4
1 2
2 3
3 4
4 1
1 3

Sample Output 1

2

Ken can reach Vertex 3 from Vertex 1 in two ken-ken-pa, as follows: 1 \rightarrow 2 \rightarrow 3 \rightarrow 4 in the first ken-ken-pa, then 4 \rightarrow 1 \rightarrow 2 \rightarrow 3 in the second ken-ken-pa. This is the minimum number of ken-ken-pa needed.

Sample Input 2

3 3
1 2
2 3
3 1
1 2

Sample Output 2

-1

Any number of ken-ken-pa will bring Ken back to Vertex 1, so he cannot reach Vertex 2, though he can pass through it in the middle of a ken-ken-pa.

Sample Input 3

2 0
1 2

Sample Output 3

-1

Vertex S and Vertex T may be disconnected.

Sample Input 4

6 8
1 2
2 3
3 4
4 5
5 1
1 4
1 5
4 6
1 6

Sample Output 4

2

Solution in Haskell (GHC 7.10.3) (judge verdict: Time Limit Exceeded)

-- https://github.com/minoki/my-atcoder-solutions
{-# LANGUAGE BangPatterns #-}
import Data.Char (isSpace)
import Data.List (unfoldr)
import Control.Monad
import qualified Data.Vector as V
import qualified Data.Vector.Mutable as VM
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.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 graph1 :: V.Vector IntSet.IntSet
      graph1 = V.create $ do
        vec <- VM.replicate n IntSet.empty
        U.forM_ edges $ \(u,v) -> do
          s <- VM.read vec u
          VM.write vec u $! IntSet.insert v s
        return vec
      graph2 :: V.Vector IntSet.IntSet
      graph2 = V.create $ do
        vec <- VM.new n
        let go 0 !u = IntSet.singleton u
            go 1 !u = graph1 V.! u
            go i !u = foldMap_IntSet (go (i - 1)) $ graph1 V.! u
        forM_ [0..n-1] $ \u -> do
          VM.write vec u $! go 3 u
        return vec
      result = runST $ do
        visited <- UM.replicate n False
        -- bfs :: IntSet -> ST s (Maybe Int)
        let bfs !depth ss = do
              forM_IntSet ss $ \u ->
                UM.write visited u True
              let ts = foldMap_IntSet (graph2 V.!) ss
              ts' <- foldMapM_IntSet (\v -> do
                                         d <- UM.read visited v
                                         pure $ if d then IntSet.empty else IntSet.singleton v
                                     ) ts
              if t `IntSet.member` ts'
                then return (Just depth)
                else if IntSet.null ts'
                     then return Nothing
                     else bfs (depth+1) ts'
        bfs 1 (IntSet.singleton s)
  case result of
    Nothing -> putStrLn "-1"
    Just d -> print d

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