

import java.util.Scanner;
import java.util.Stack;

public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner sc = new Scanner(System.in);

		char[] str = sc.next().toCharArray();

		Stack<Character> stack = new Stack<Character>();

//		int count = 0;
//		for(int i = 0; i < str.length; i++){
//			if(str[i] == '0'){
//				stack.push(0);
//			}
//			else {
//				if(!stack.isEmpty()){
//					stack.pop();
//					count++;
//				}
//			}
//		}
//
//		stack.removeAllElements();
//		int count2 = 0;
//		for(int i = 0; i < str.length; i++){
//			if(str[i] == '1'){
//				stack.push(0);
//			}
//			else {
//				if(!stack.isEmpty()){
//					stack.pop();
//					count2++;
//				}
//			}
//		}
//
//		System.out.println(Math.max(count*2, count2*2));

		int count = 0;

		for(int i = 0; i < str.length; i++){
			if(!stack.isEmpty() && stack.peek() != str[i]){
				stack.pop();
				count++;
			}
			else {
				stack.push(str[i]);
			}
		}

		System.out.println(count*2);
	}
}
/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