import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String S = scanner.next();
        Long countA = 0L;
        Long countB = 0L;
        for (int i = 0; i < S.length(); i++) {
            if(S.charAt(i) == '0') countA++;
            if(S.charAt(i) == '1') countB++;
        }
        countA = Math.min(countA, countB) * 2;
        System.out.println(countA);
    }
}
