import java.util.Scanner;
import java.util.ArrayDeque;

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

    while (in.hasNext()) {
      String S = in.next();
      ArrayDeque<Character> stack = new ArrayDeque<Character>();

      for (int i = 0; i < S.length(); ++i) {
        char cube = S.charAt(i);
        if (stack.isEmpty()) {
          stack.push(cube);
        } else if (cube == '1' && stack.peek() == '0') {
          stack.pop();
        } else if (cube == '0' && stack.peek() == '1') {
          stack.pop();
        } else {
          stack.push(cube);
        }
      }

      System.out.println(S.length() - stack.size());
    }
  }
}
/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