import Control.Applicative
import Data.ByteString.Char8 (ByteString)
import qualified Data.ByteString.Char8 as B

main :: IO ()
main = solve <$> B.words <$> B.getLine >>= print

solve :: [ByteString] -> Int
solve [bs] = let l = B.length bs
                 z = B.length $ B.filter (== '0') bs
                 o = l - z
             in 2 * min z o
