import java.util.*

fun main(args: Array<String>) {

    val sc = Scanner(System.`in`)
    val S = sc.next()
    var red = 0
    var blue= 0
    for (c in S) {
        when (c - '0') {
           0->red++
           1->blue++
        }
    }
    println(2*Math.min(red,blue))
}