import java.util.Scanner;

public class Main {
  public static void main(String[] args) {
    // 入力
    try (Scanner sc = new Scanner(System.in);) {
      String line = sc.nextLine();
      String tmp = line;
      int zero = 0;
      int one = 0;
      for (int i = 0; i < tmp.length(); i++) {
        if(tmp.charAt(i) == '0') {
          zero++;
        }
        if(tmp.charAt(i) == '1') {
          one++;
        }
      }
      System.out.println(2 * Math.min(zero, one));
    }
  }
}
/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