import java.util.*

object MainKt {
	@JvmStatic
	fun main(args: Array<String>) {
		val scanner = Scanner(System.`in`)
		val s = scanner.next()

		val zeroCount = count(s, '0')
		val oneCount = s.length - zeroCount

		println(Math.min(zeroCount, oneCount)*2)
	}

	fun count(str: String, target: Char): Int {
		var count = 0
		str.toCharArray().forEach { c -> if (c==target) count++ }
		return count
	}
}