import java.util.LinkedList;
import java.util.Scanner;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String S = sc.next();
    Main abc120C = new Main();
    System.out.println(abc120C.solve(S));
  }

  public int solve(String S) {
    ABC120CSolve abc120CSolve = new ABC120CSolve();
    return abc120CSolve.solve(S);
  }

  class ABC120CSolve {
    static final int ZERO = (int) '0';

    public int solve(String S) {
      int ans = 0;
      LinkedList<Integer> list = new LinkedList();

      for (char c : S.toCharArray()) {
        int i = (int) c - ZERO;
        if (list.size() == 0) {
          list.addLast(i);
        } else {
          if (list.getLast() != i) {
            list.removeLast();
            ans += 2;
          } else {
            list.addLast(i);
          }
        }
      }
      return ans;
    }
  }
}

/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