{-# LANGUAGE BangPatterns #-}
{-# LANGUAGE MultiWayIf #-}
{-# LANGUAGE TypeFamilies #-}

module Main where

import           Control.Monad
import           Control.Monad.Extra
import           Control.Monad.Primitive
import qualified Data.Array as A
import qualified Data.Array.IO as AIO
import qualified Data.Array.Unboxed as AU
import           Data.Bits
import           Data.Bool
import qualified Data.ByteString as BS
import qualified Data.ByteString.Char8 as BSC8
import           Data.Char
import           Data.Either
import           Data.Function
import qualified Data.Graph.Inductive.Graph as G
import qualified Data.Graph.Inductive.PatriciaTree as G
import qualified Data.Heap as H
import           Data.Int
import           Data.List
import qualified Data.Map.Strict as M
import           Data.Maybe
import           Data.Monoid
import           Data.Ord
import           Data.Semigroup
import qualified Data.Sequence as SQ
import qualified Data.Set as S
import           Data.Time
import           Data.Tuple
import qualified Data.Vector as V
import           Data.Vector.Algorithms.Search
import qualified Data.Vector.Mutable as VM
import qualified Data.Vector.Unboxed as VU
import qualified Data.Vector.Unboxed.Mutable as VUM
import qualified Data.Vector.Unboxing as VUG
import qualified Data.Vector.Unboxing.Mutable as VUGM
import           Debug.Trace
import           GHC.Stack

main :: IO ()
main = do
  --aN       <- getI
  --[aA, aB] <- getIL
  --aS       <- getLine
  [aN, aM] <- getIL
  aAB <- VU.replicateM aM $ getIT2
  dsuTable <- aclDsuTable aN
  VU.mapM_ (\(a, b) -> aclDsuMerge dsuTable (pred a) (pred b)) aAB
  aa <- aclDsuGroup dsuTable
  print $ V.length aa - 1
  return ()

--ACL(AtCoder LibraryのHaskell移植版(おっさんたたき台Ver.))
--DSU
aclDsuTable n = do
  parentOrSize <- VUM.replicate n (-1)
  return (n, parentOrSize)

aclDsuMerge table@(n, parentOrSize) a b
  | a < 0
    || n <= a = error "aclDsuMerge:aの範囲を見直してください。"
  | b < 0
    || n <= b = error "aclDsuMerge:bの範囲を見直してください。"
  | otherwise = do
    x <- aclDsuLeader table a
    y <- aclDsuLeader table b
    if x == y
      then return x
      else do
        posX <- VUM.read parentOrSize x
        posY <- VUM.read parentOrSize y
        if -posX < -posY
          then do
            VUM.write parentOrSize y (posY + posX)
            VUM.write parentOrSize x y
            return y
          else do
            VUM.write parentOrSize x (posX + posY)
            VUM.write parentOrSize y x
            return x

aclDsuSame table@(n, parentOrSize) a b
  | a < 0
    && n <= a = error "aclDsuSame:aの範囲を見直してください。"
  | b < 0
    && n <= b = error "aclDsuSame:bの範囲を見直してください。"
  | otherwise = do
    x <- aclDsuLeader table a
    y <- aclDsuLeader table b
    return $ x == y

aclDsuLeader table@(n, parentOrSize) a
  | a < 0
    && n <= a = error "aclDsuLeader:aの範囲を見直してください。"
  | otherwise = do
    posA <- VUM.read parentOrSize a
    if posA < 0
      then return a
      else do
        x <- aclDsuLeader table posA
        VUM.write parentOrSize a x
        return x

aclDsuSize table@(n, parentOrSize) a
  | a < 0
    && n <= a = error "aclDsuSize:aの範囲を見直してください。"
  | otherwise = do
    x <- aclDsuLeader table a
    negate <$> VUM.read parentOrSize x

aclDsuGroup table@(n, parentOrSize) = do
  leaderBuf <- VUM.replicate n 0 :: IO (VUM.IOVector Int)
  groupSize <- VUM.replicate n 0 :: IO (VUM.IOVector Int)
  VU.forM_ (VU.enumFromN 0 n)
    $ \i -> do
      leaderI <- aclDsuLeader table i
      VUM.write leaderBuf i leaderI
      VUM.modify groupSize succ leaderI
  resultTmpIndex <- VUM.replicate n 0 :: IO (VUM.IOVector Int)
  resultTmp <- (V.generateM n
                $ \i -> do
                  gs <- VUM.read groupSize i
                  VUM.replicate gs 0 :: IO (VUM.IOVector Int))
  VU.forM_ (VU.enumFromN 0 n)
    $ \i -> do
      leaderI <- VUM.read leaderBuf i
      tmpIndex <- VUM.read resultTmpIndex leaderI
      VUM.write (resultTmp V.! leaderI) tmpIndex i
      VUM.modify resultTmpIndex succ leaderI
  V.mapMaybe id
    <$> (V.mapM
           (\i -> do
              rti <- VU.freeze $ resultTmp V.! i :: IO (VU.Vector Int)
              if rti == VU.empty
                then return Nothing
                else return $ Just rti)
         $ V.enumFromN 0 n)

--雑多系
t :: Int -> (Int, Int)
t x = (x, 1)

i def f t = bool t def $ f t

shiftRCnt k = shiftRCntP k 1

shiftRCntP k c = let nk = shiftR k 1
                 in if nk == 0
                    then c
                    else shiftRCntP nk (c + 1)

myVUmap f vu =
  if vu == VU.empty
  then VU.empty
  else VU.unfoldr
    (\x -> let ret = f $ VU.head x
               tailx = VU.tail x
           in if x == VU.empty
              then Nothing
              else if isLeft ret
                   then Just (either id id ret, tailx)
                   else Just (either id id ret, VU.empty))
    vu

myVUmapM f vu =
  if vu == VU.empty
  then return VU.empty
  else VU.unfoldrM
    (\x -> do
       ret <- f $ VU.head x
       let tailx = VU.tail x
       if x == VU.empty
         then return Nothing
         else if isLeft ret
              then return $ Just (either id id ret, tailx)
              else return $ Just (either id id ret, VU.empty))
    vu

myVUmapM_ f vu =
  if vu == VU.empty
  then return ()
  else do
    ret1 <- VU.unfoldrM
      (\x -> do
         ret2 <- f $ VU.head x
         let tailx = VU.tail x
         if x == VU.empty
           then return Nothing
           else if isLeft ret2
                then return $ Just (either id id ret2, tailx)
                else return $ Just (either id id ret2, VU.empty))
      vu
    return ()

myVUfor vu f = myVUmap f vu

myVUforM vu f = myVUmapM f vu

myVUforM_ vu f = myVUmapM_ f vu

myVUfoldl f acc vu =
  if vu == VU.empty
  then return acc
  else do
    let ret = f acc (VU.head vu)
    if isLeft ret
      then myVUfoldl f (either id id ret) (VU.tail vu)
      else return $ either id id ret

myVUfoldr f acc vu =
  if vu == VU.empty
  then return acc
  else do
    let ret = f acc (VU.last vu)
    if isLeft ret
      then myVUfoldr f (either id id ret) (VU.init vu)
      else return $ either id id ret

myVUfoldM f acc vu =
  if vu == VU.empty
  then return acc
  else do
    ret <- f acc (VU.head vu)
    if isLeft ret
      then myVUfoldM f (either id id ret) (VU.tail vu)
      else return $ either id id ret

--周りのマス
aroundSquare y x h w = VU.concatMap
  (\a -> VU.mapMaybe
     (\b -> let ya = y + a
                xb = x + b
            in if abs a + abs b == 1
               then if ya < 0 || ya >= h || xb < 0 || xb >= w
                    then Nothing
                    else Just (ya, xb)
               else Nothing)
   $ VU.enumFromN (-1) 3)
  $ VU.enumFromN (-1) 3

--入力系
splitReadBSC8 :: BSC8.ByteString -> [Int]
splitReadBSC8 bs = let (bs1, newBs) = BSC8.splitAt 1 bs
                   in if newBs == BSC8.empty
                      then [test $ BSC8.readInt bs1]
                      else test (BSC8.readInt bs1):splitReadBSC8 newBs
  where
    test (Just (i, _)) = i

read2DimChar :: VM.IOVector (VUM.IOVector Char) -> Int -> Int -> IO Char
read2DimChar vec y x = do
  vecY <- VM.read vec y
  VUM.read vecY x

write2DimChar :: VM.IOVector (VUM.IOVector Char) -> Int -> Int -> Char -> IO ()
write2DimChar vec y x t = do
  vecY <- VM.read vec y
  VUM.write vecY x t

readTwoDimVec :: VM.IOVector (VUM.IOVector Int) -> Int -> Int -> IO Int
readTwoDimVec vec y x = do
  vecY <- VM.read vec y
  VUM.read vecY x

writeTwoDimVec :: VM.IOVector (VUM.IOVector Int) -> Int -> Int -> Int -> IO ()
writeTwoDimVec vec y x t = do
  vecY <- VM.read vec y
  VUM.write vecY x t

getVUI :: IO (VU.Vector Int)
getVUI = VU.unfoldr (BSC8.readInt . BSC8.dropWhile isSpace) <$> BSC8.getLine

getRepVUI :: Int -> IO (VU.Vector Int)
getRepVUI n = VU.unfoldr (BSC8.readInt . BSC8.dropWhile isSpace) . BSC8.unlines
  <$> replicateM n BSC8.getLine

getVS :: IO (V.Vector Char)
getVS = V.fromList <$> getLine

getVUC :: IO (VU.Vector Char)
getVUC = VU.fromList <$> getLine

readI :: BSC8.ByteString -> Int
readI = fst . fromJust . BSC8.readInt

readInteger :: BSC8.ByteString -> Integer
readInteger = fst . fromJust . BSC8.readInteger

readIL :: BSC8.ByteString -> [Int]
readIL = map readI . BSC8.words

readIT2 :: BSC8.ByteString -> (Int, Int)
readIT2 bs = let words = BSC8.words bs
             in (readI $ head words, readI $ last words)

readIT3 :: BSC8.ByteString -> (Int, Int, Int)
readIT3 bs = let words = BSC8.words bs
             in (readI $ head words, readI $ words !! 1, readI $ words !! 2)

getI :: IO Int
getI = readI <$> BSC8.getLine

getInteger :: IO Integer
getInteger = readInteger <$> BSC8.getLine

getIL :: IO [Int]
getIL = readIL <$> BSC8.getLine

getIT2 :: IO (Int, Int)
getIT2 = readIT2 <$> BSC8.getLine

getIT3 :: IO (Int, Int, Int)
getIT3 = readIT3 <$> BSC8.getLine

getIT2Order :: IO (Int, Int)
getIT2Order = getIL
  >>= \x@[xz, xo] -> if xz > xo
                     then return (xo, xz)
                     else return (xz, xo)

getNIS :: Int -> IO [(Int, BSC8.ByteString)]
getNIS 0 = return []
getNIS n = do
  [aFst, aSnd] <- BSC8.words <$> BSC8.getLine
  next <- getNIS (n - 1)
  return ((readI aFst, aSnd):next)

il2Words :: [Int] -> String
il2Words = unwords . map show

il2Lines :: [Int] -> String
il2Lines = unlines . map show

Measured memory_bytes: 23792