Problem:
Score : 300 points

Problem Statement

There are N cubes stacked vertically on a desk.

You are given a string S of length N. The color of the i-th cube from the bottom is red if the i-th character in S is 0, and blue if that character is 1.

You can perform the following operation any number of times: choose a red cube and a blue cube that are adjacent, and remove them. Here, the cubes that were stacked on the removed cubes will fall down onto the object below them.

At most how many cubes can be removed?

Constraints

1 \leq N \leq 10^5

|S| = N

Each character in S is 0 or 1.

Input

Input is given from Standard Input in the following format:

S

Output

Print the maximum number of cubes that can be removed.

Sample Input 1

0011

Sample Output 1

4

All four cubes can be removed, by performing the operation as follows:

Remove the second and third cubes from the bottom. Then, the fourth cube drops onto the first cube.

Remove the first and second cubes from the bottom.

Sample Input 2

11011010001011

Sample Output 2

12

Sample Input 3

0

Sample Output 3

0

Solution in Haskell (GHC 7.10.3) (judge verdict: Accepted)

-- stack script --resolver lts-14.6
{-# LANGUAGE CPP #-}
module Main where

import qualified Data.Vector as V 
import qualified Data.Map.Strict as M
import Control.Monad
import qualified Data.ByteString.Char8 as BS
import Data.Maybe

dump :: (Show a) => a -> IO()
#if __GLASGOW_HASKELL__ >= 800
dump x = print x
#else
dump _ = pure ()
#endif

readint :: IO Int
readint = (fst . fromJust . BS.readInt) <$> BS.getLine

readvec :: IO [Int]
readvec = (fmap (read . BS.unpack) . BS.words) <$> BS.getLine

-- "101010" -> [1,0,1,0,1,0]
readToList :: String -> [Int]
readToList [] = []
readToList (x:xs) = read [x] : readToList xs

newtype UF a = UF (M.Map a a) deriving (Show) -- Map node parent

makeuf :: Int -> UF Int
makeuf n = let m = M.fromList $ zip [0..n] [0..n]
                in UF m

union :: (Eq a, Ord a) => a -> a -> UF a -> UF a
union l r uf@(UF m) = let lp = find l uf
                          rp = find r uf
                      in if lp == rp
                         then uf
                         else if size l uf < size r uf
                              then UF $ M.update (const $ Just rp) lp m
                              else UF $ M.update (const $ Just lp) rp m

find :: (Eq a, Ord a) => a -> UF a -> a
find x uf@(UF m) = let px = m M.! x
                in if px == x
                   then px
                   else find px uf

size :: (Eq a, Ord a) => a -> UF a -> Int
size x (UF m) = M.size $ M.filter (\par -> par==x) m

main :: IO()
main = do
  s <-  readToList . BS.unpack <$> BS.getLine :: IO [Int]
  let count x = length $ filter (== x) s
  print $ (* 2) $ min (count 0) (count 1)