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

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

    Stack<Character> stack = new Stack<>();
    char[] s = scanner.next().toCharArray();
    int N = s.length;
    for (char c : s) {
      if (!stack.isEmpty() && stack.peek() != c) stack.pop();
      else stack.add(c);
    }
    System.out.println(N - 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