import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String input = sc.nextLine();

		int count = 0;
		String inputStr[] = new String[input.length()];
		int currentIndex = 0;
		for(int i = 0; i < input.length(); i++) {
			inputStr[currentIndex]= input.substring(i,i+1);
			if(currentIndex!=0 && !inputStr[currentIndex].equals(inputStr[currentIndex-1])) {
				currentIndex--;
				count+=2;
			}else {
				currentIndex++;
			}
		}
		System.out.println(count);
		sc.close();
	}
}

/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz