{-# OPTIONS_GHC -O2 #-}
{-# LANGUAGE BangPatterns, BinaryLiterals, CPP, DerivingStrategies  #-}
{-# LANGUAGE DerivingVia, FlexibleContexts, FlexibleInstances       #-}
{-# LANGUAGE GeneralizedNewtypeDeriving, KindSignatures, LambdaCase #-}
{-# LANGUAGE MagicHash, MultiParamTypeClasses, MultiWayIf           #-}
{-# LANGUAGE NumericUnderscores, OverloadedStrings, PatternSynonyms #-}
{-# LANGUAGE RankNTypes, RecordWildCards, ScopedTypeVariables       #-}
{-# LANGUAGE StandaloneDeriving, TupleSections, TypeApplications    #-}
{-# LANGUAGE TypeFamilies, TypeInType, UnboxedTuples, ViewPatterns  #-}

import           Control.Applicative
import           Control.Exception
import           Control.Monad
import           Control.Monad.Primitive
import           Control.Monad.Reader
import           Control.Monad.ST
import           Control.Monad.State.Strict
import           Data.Bifunctor
import           Data.Bool
import qualified Data.ByteString               as B
import qualified Data.ByteString.Builder       as B
import qualified Data.ByteString.Char8         as C
import qualified Data.ByteString.Internal      as B
import qualified Data.ByteString.Unsafe        as B
import           Data.Char
import qualified Data.Foldable                 as F
import           Data.Function
import           Data.Functor.Identity
import qualified Data.IntMap.Strict            as IM
import qualified Data.IntSet                   as IS
import qualified Data.List                     as L
import qualified Data.Map.Strict               as M
import           Data.Monoid
import           Data.Ord
import           Data.Primitive
import           Data.Proxy
import           Data.Ratio
import           Data.Semigroup
import qualified Data.Set                      as S
import           Data.Tuple
import qualified Data.Vector                   as V
import qualified Data.Vector.Generic           as G
import qualified Data.Vector.Generic.Mutable   as GM
import qualified Data.Vector.Mutable           as VM
import qualified Data.Vector.Primitive         as P
import qualified Data.Vector.Primitive.Mutable as PM
import qualified Data.Vector.Unboxed           as U
import qualified Data.Vector.Unboxed.Mutable   as UM
import           Debug.Trace
import           Foreign                       hiding (void)
import           GHC.Exts
import           GHC.TypeLits
import qualified System.IO                     as IO
import           Unsafe.Coerce

#define MOD 1000000007

main :: IO ()
main = do
    [n, u, v] <- map read.words <$> getLine :: IO [Int]
    es <- U.unfoldrN (n - 1) (runParser $ (,) <$> int1 <*> int1) <$> C.getContents
    print $ solve n (u-1) (v-1) es

solve :: Int -> Vertex -> Vertex -> U.Vector (Vertex, Vertex) -> Int
solve n u0 v0 es
    = end
    . U.maximum
    . U.map (queryDist lca u1)
    . U.filter (\x -> queryDepth lca u1 <= queryDepth lca x && queryDist lca x u1 < queryDist lca x v1)
    $ U.generate n id
  where
    !gr = buildUndirectedGraph n es
    !lca = buildLCA gr v0
    !d0 = queryDist lca u0 v0
    !q = quot (d0 - 1) 2
    p du dv x = queryDist lca u0 x == du && queryDist lca v0 x == dv
    !u1 | even d0 = U.head . U.filter (p q (q + 2)) $ U.generate n id
        | otherwise = U.head . U.filter (p q (q + 1)) $ U.generate n id
    !v1 | even d0 = U.head . U.filter (p (q + 2) q) $ U.generate n id
        | otherwise = U.head . U.filter (p (q + 1) q) $ U.generate n id
    !d1 = queryDist lca u1 v1
    end du1
        | d1 == 1 = du1 + q
        | otherwise = du1 + 1 + q

floorSqrt :: Int -> Int
floorSqrt = floor . sqrt . fromIntegral

floorLog2 :: Int -> Int
floorLog2 x = fromIntegral $ unsafeShiftR y 52 - 1023
  where
    y :: Word64
    y = unsafeCoerce (fromIntegral x :: Double)

-------------------------------------------------------------------------------
-- Utils
-------------------------------------------------------------------------------
rep :: Monad m => Int -> (Int -> m ()) -> m ()
rep !n = U.forM_ $ U.generate n id
{-# INLINE rep #-}
rev :: Monad m => Int -> (Int -> m ()) -> m ()
rev !n = U.forM_ $ U.iterateN n (subtract 1) (n - 1)
{-# INLINE rev #-}
infixl 8 `shiftRL`, `unsafeShiftRL`
shiftRL = unsafeShiftRL
{-# INLINE shiftRL #-}
unsafeShiftRL (I# x#) (I# i#) = I# (uncheckedIShiftRL# x# i#)
{-# INLINE unsafeShiftRL #-}
type Parser a = StateT C.ByteString Maybe a
runParser :: Parser a -> C.ByteString -> Maybe (a, C.ByteString)
runParser = runStateT
{-# INLINE runParser #-}
int :: Parser Int
int = coerce $ C.readInt . C.dropWhile isSpace
{-# INLINE int #-}
int1 :: Parser Int
int1 = fmap (subtract 1) int
{-# INLINE int1 #-}
char :: Parser Char
char = coerce C.uncons
{-# INLINE char #-}
byte :: Parser Word8
byte = coerce B.uncons
{-# INLINE byte #-}
-------------------------------------------------------------------------------
-- Data.VecQueue
-------------------------------------------------------------------------------
data VecQueue s a = VecQueue{intVarsVQ :: !(UM.MVector s Int), internalVecQueue :: !(UM.MVector s a)}
_dequeueCount :: Int
_dequeueCount = 0
{-# INLINE _dequeueCount #-}
_enqueueCount :: Int
_enqueueCount = 1
{-# INLINE _enqueueCount #-}
newVecQueue :: (PrimMonad m, UM.Unbox a) => Int -> m (VecQueue (PrimState m) a)
newVecQueue n = VecQueue <$> UM.replicate 2 0 <*> UM.unsafeNew n
defaultVecQueueSize :: Int
defaultVecQueueSize = 1024 * 1024
lengthVQ :: (PrimMonad m, UM.Unbox a) => VecQueue (PrimState m) a -> m Int
lengthVQ (VecQueue info _) = (-) <$> UM.unsafeRead info _enqueueCount <*> UM.unsafeRead info _dequeueCount
{-# INLINE lengthVQ #-}
dequeueVQ :: (PrimMonad m, UM.Unbox a) => VecQueue (PrimState m) a -> m (Maybe a)
dequeueVQ (VecQueue info q) = do { f <- UM.unsafeRead info _dequeueCount; r <- UM.unsafeRead info _enqueueCount; if f < r then do { UM.unsafeWrite info _dequeueCount (f + 1); pure <$> UM.unsafeRead q f} else return Nothing}
{-# INLINE dequeueVQ #-}
enqueueVQ :: (PrimMonad m, UM.Unbox a) => a -> VecQueue (PrimState m) a -> m ()
enqueueVQ x (VecQueue info q) = do { r <- UM.unsafeRead info _enqueueCount; UM.unsafeWrite q r x; UM.unsafeWrite info _enqueueCount (r + 1)}
{-# INLINE enqueueVQ #-}
enqueuesVQ :: (PrimMonad m, UM.Unbox a) => U.Vector a -> VecQueue (PrimState m) a -> m ()
enqueuesVQ vec (VecQueue info q) = do { r <- UM.unsafeRead info _enqueueCount; UM.unsafeWrite info _enqueueCount (r + U.length vec); U.unsafeCopy (UM.unsafeSlice r (U.length vec) q) vec}
{-# INLINE enqueuesVQ #-}
clearVQ :: (UM.Unbox a, PrimMonad m) => VecQueue (PrimState m) a -> m ()
clearVQ (VecQueue info _) = do { UM.unsafeWrite info _dequeueCount 0; UM.unsafeWrite info _enqueueCount 0}
freezeVecQueue :: (PrimMonad m, UM.Unbox a) => VecQueue (PrimState m) a -> m (U.Vector a)
freezeVecQueue (VecQueue info q) = do { f <- UM.unsafeRead info _dequeueCount; r <- UM.unsafeRead info _enqueueCount; U.unsafeFreeze $ UM.unsafeSlice f (r - f) q}
-------------------------------------------------------------------------------
-- Data.Graph.Sparse
-------------------------------------------------------------------------------
type Vertex = Int
type Edge = (Vertex, Vertex)
type EdgeWith w = (Vertex, Vertex, w)
type EdgeId = Int
data SparseGraph w = CSR{numVerticesCSR :: !Int, numEdgesCSR :: !Int, offsetCSR :: !(U.Vector Int), adjacentCSR :: !(U.Vector Vertex), edgeCtxCSR :: !(U.Vector w)}
data SparseGraphBuilder s w = SparseGraphBuilder{numVerticesSGB :: !Int, queueSGB :: VecQueue s (EdgeWith w), outDegSGB :: UM.MVector s Int}
buildSparseGraph :: (U.Unbox w) => Int -> (forall s . SparseGraphBuilder s w -> ST s ()) -> SparseGraph w
buildSparseGraph numVerticesCSR run = runST $ do { queueSGB <- newVecQueue (1024 * 1024); outDegSGB <- UM.replicate numVerticesCSR 0; run SparseGraphBuilder{numVerticesSGB = numVerticesCSR, ..}; numEdgesCSR <- lengthVQ queueSGB; offsetCSR <- U.scanl' (+) 0 <$> U.unsafeFreeze outDegSGB; moffset <- U.thaw offsetCSR; madj <- UM.unsafeNew numEdgesCSR; mectx <- UM.unsafeNew numEdgesCSR; edges <- freezeVecQueue queueSGB; U.forM_ edges $ \ (src, dst, w) -> do { pos <- UM.unsafeRead moffset src; UM.unsafeWrite moffset src (pos + 1); UM.unsafeWrite madj pos dst; UM.unsafeWrite mectx pos w}; adjacentCSR <- U.unsafeFreeze madj; edgeCtxCSR <- U.unsafeFreeze mectx; return CSR{..}}
{-# INLINE buildSparseGraph #-}
addDirectedEdge :: (U.Unbox w, PrimMonad m) => SparseGraphBuilder (PrimState m) w -> EdgeWith w -> m ()
addDirectedEdge SparseGraphBuilder{..} (src, dst, w) = do { enqueueVQ (src, dst, w) queueSGB; UM.unsafeModify outDegSGB (+ 1) src}
{-# INLINE addDirectedEdge #-}
addUndirectedEdge :: (U.Unbox w, PrimMonad m) => SparseGraphBuilder (PrimState m) w -> EdgeWith w -> m ()
addUndirectedEdge SparseGraphBuilder{..} (src, dst, w) = do { enqueueVQ (src, dst, w) queueSGB; enqueueVQ (dst, src, w) queueSGB; UM.unsafeModify outDegSGB (+ 1) src; UM.unsafeModify outDegSGB (+ 1) dst}
{-# INLINE addUndirectedEdge #-}
addDirectedEdge_ :: (PrimMonad m) => SparseGraphBuilder (PrimState m) () -> Edge -> m ()
addDirectedEdge_ SparseGraphBuilder{..} (src, dst) = do { enqueueVQ (src, dst, ()) queueSGB; UM.unsafeModify outDegSGB (+ 1) src}
{-# INLINE addDirectedEdge_ #-}
addUndirectedEdge_ :: (PrimMonad m) => SparseGraphBuilder (PrimState m) () -> Edge -> m ()
addUndirectedEdge_ SparseGraphBuilder{..} (src, dst) = do { enqueueVQ (src, dst, ()) queueSGB; enqueueVQ (dst, src, ()) queueSGB; UM.unsafeModify outDegSGB (+ 1) src; UM.unsafeModify outDegSGB (+ 1) dst}
{-# INLINE addUndirectedEdge_ #-}
buildDirectedGraph :: Int -> U.Vector Edge -> SparseGraph ()
buildDirectedGraph numVerticesCSR edges = buildSparseGraph numVerticesCSR $ \ builder -> do { U.mapM_ (addDirectedEdge_ builder) edges}
buildUndirectedGraph :: Int -> U.Vector Edge -> SparseGraph ()
buildUndirectedGraph numVerticesCSR edges = buildSparseGraph numVerticesCSR $ \ builder -> do { U.mapM_ (addUndirectedEdge_ builder) edges}
buildDirectedGraphW :: (U.Unbox w) => Int -> U.Vector (EdgeWith w) -> SparseGraph w
buildDirectedGraphW numVerticesCSR edges = buildSparseGraph numVerticesCSR $ \ builder -> do { U.mapM_ (addDirectedEdge builder) edges}
buildUndirectedGraphW :: (U.Unbox w) => Int -> U.Vector (EdgeWith w) -> SparseGraph w
buildUndirectedGraphW numVerticesCSR edges = buildSparseGraph numVerticesCSR $ \ builder -> do { U.mapM_ (addUndirectedEdge builder) edges}
adj :: SparseGraph w -> Vertex -> U.Vector Vertex
adj CSR{..} v = U.unsafeSlice o (o' - o) adjacentCSR where { o = U.unsafeIndex offsetCSR v; o' = U.unsafeIndex offsetCSR (v + 1)}
{-# INLINE adj #-}
iadj :: SparseGraph w -> Vertex -> U.Vector (EdgeId, Vertex)
iadj CSR{..} v = U.imap ((,) . (+ o)) $ U.unsafeSlice o (o' - o) adjacentCSR where { o = U.unsafeIndex offsetCSR v; o' = U.unsafeIndex offsetCSR (v + 1)}
{-# INLINE iadj #-}
adjW :: (U.Unbox w) => SparseGraph w -> Vertex -> U.Vector (Vertex, w)
adjW CSR{..} v = U.zip (U.unsafeSlice o (o' - o) adjacentCSR) (U.unsafeSlice o (o' - o) edgeCtxCSR) where { o = U.unsafeIndex offsetCSR v; o' = U.unsafeIndex offsetCSR (v + 1)}
{-# INLINE adjW #-}
iadjW :: (U.Unbox w) => SparseGraph w -> Vertex -> U.Vector (EdgeId, Vertex, w)
iadjW CSR{..} v = U.izipWith (\ i u w -> (i + o, u, w)) (U.unsafeSlice o (o' - o) adjacentCSR) (U.unsafeSlice o (o' - o) edgeCtxCSR) where { o = U.unsafeIndex offsetCSR v; o' = U.unsafeIndex offsetCSR (v + 1)}
{-# INLINE iadjW #-}
outEdges :: SparseGraph w -> Vertex -> U.Vector EdgeId
outEdges CSR{..} v = U.generate (o' - o) (+ o) where { o = U.unsafeIndex offsetCSR v; o' = U.unsafeIndex offsetCSR (v + 1)}
{-# INLINE outEdges #-}
outDegree :: SparseGraph w -> Vertex -> Int
outDegree CSR{..} v = U.unsafeIndex offsetCSR (v + 1) - U.unsafeIndex offsetCSR v
{-# INLINE outDegree #-}
outDegrees :: SparseGraph w -> U.Vector Int
outDegrees CSR{..} = U.zipWith (-) offsetCSR $ U.tail offsetCSR
{-# INLINE outDegrees #-}
-------------------------------------------------------------------------------
-- Data.Graph.Tree.LCA
-------------------------------------------------------------------------------
data LCA = LCA{firstIndexLCA :: U.Vector Int, rmqLCA :: RMQ (Int, Vertex)}
buildLCA :: (U.Unbox w) => SparseGraph w -> Vertex -> LCA
buildLCA gr root = runST $ do { met <- newVecQueue (2 * numVerticesCSR gr - 1); mfv <- UM.unsafeNew (numVerticesCSR gr); fix (\ dfs !p !d !v -> do { lengthVQ met >>= UM.unsafeWrite mfv v; enqueueVQ (d, v) met; U.forM_ (gr `adj` v) $ \ nv -> do { when (nv /= p) $ do { dfs v (d + 1) nv; enqueueVQ (d, v) met}}}) (-1) 0 root; eulertour <- freezeVecQueue met; firstVisit <- U.unsafeFreeze mfv; pure $ LCA firstVisit (buildRMQ eulertour)}
queryLCA :: LCA -> Vertex -> Vertex -> Vertex
queryLCA LCA{..} v u = snd $ queryMin rmqLCA (min i j) (max i j + 1) where { !i = U.unsafeIndex firstIndexLCA v; !j = U.unsafeIndex firstIndexLCA u}
{-# INLINE queryLCA #-}
queryDepth :: LCA -> Vertex -> Int
queryDepth LCA{..} v = fst . readRMQ rmqLCA $ U.unsafeIndex firstIndexLCA v
{-# INLINE queryDepth #-}
queryDist :: LCA -> Vertex -> Vertex -> Int
queryDist lca v u = queryDepth lca v + queryDepth lca u - 2 * queryDepth lca (queryLCA lca v u)
{-# INLINE queryDist #-}
-------------------------------------------------------------------------------
-- Data.SparseTable
-------------------------------------------------------------------------------
type RMQ a = SparseTable Min a
buildRMQ :: (U.Unbox a, Ord a) => U.Vector a -> RMQ a
buildRMQ = buildSparseTable
{-# INLINE buildRMQ #-}
readRMQ :: (U.Unbox a) => RMQ a -> Int -> a
readRMQ = readSparseTable
{-# INLINE readRMQ #-}
queryMin :: (U.Unbox a, Ord a) => RMQ a -> Int -> Int -> a
queryMin = querySparseTable
{-# INLINE queryMin #-}
newtype SparseTable (f :: * -> *) a = SparseTable{getSparseTable :: V.Vector (U.Vector a)} deriving (Eq, Show)
buildSparseTable :: forall (f :: * -> *) a . (U.Unbox a, Semigroup (f a), Coercible (f a) a) => U.Vector a -> SparseTable f a
buildSparseTable vec = SparseTable . V.scanl' (\ acc i -> U.zipWith (coerce ((<>) @(f a))) acc $ U.drop i acc) vec $ V.iterateN (floorLog2 $ U.length vec) (* 2) 1
readSparseTable :: (U.Unbox a) => SparseTable f a -> Int -> a
readSparseTable st i = U.unsafeIndex (V.unsafeIndex (getSparseTable st) 0) i
querySparseTable :: forall (f :: * -> *) a . (U.Unbox a, Semigroup (f a), Coercible (f a) a) => SparseTable f a -> Int -> Int -> a
querySparseTable st l r = (coerce ((<>) @(f a))) x y where { logStep = floorLog2 $ r - l; row = V.unsafeIndex (getSparseTable st) logStep; x = U.unsafeIndex row l; y = U.unsafeIndex row $ r - unsafeShiftL 1 logStep}


Measured memory_bytes: 65256