import java.util.Scanner

import scala.annotation.tailrec
import scala.io.StdIn

object Main {
  def main(args: Array[String]) = {
    val S = StdIn.readLine()

    val (zeros, ones) = S.partition(_ == '0')
    println(zeros.length.min(ones.length) * 2)

    /*
    @tailrec
    def calc(target: Char, remains: String, pre: String = "", num: Int = 0): (String, Int) = {
      if (remains.isEmpty) (pre, num)
      else if (pre.isEmpty) calc(remains.head, remains.tail, pre + target, num)
      else {
        val (r, n) = if (pre.last != target) (pre.init, num + 2) else (pre + target, num)
        calc(remains.head, remains.tail, r, n)
      }
    }

    @tailrec
    def loop(target: String, total: Int = 0): Int = {
      val (pre, num) = calc(target.head, target.tail)
      println(pre)
      if (num == 0) total else loop(pre, total + num)
    }

    println(loop(S))
    */
  }
}
