import java.util.Scanner

object Main {
  def main(args: Array[String]): Unit = {
    //nextLineとnextを同時に使ったら死ぬ
    val sc = new Scanner(System.in)
    val s = sc.next()
    val one = s.count(_ == '1')
    val zero = s.count(_ == '0')

    println(math.min(one, zero) * 2)
  }

}
