import java.util.Scanner;

public class Main{
  static String str;
  static int N,zeroC,oneC;
  static int[] A;
  public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    str = sc.next();
    N = str.length();
    A = new int[N];
    for(int i=0;i<N;i++){
      A[i]=Character.getNumericValue(str.charAt(i));
      if(A[i]==0){
        zeroC++;
      }
      else{
        oneC++;
      }
    }
    System.out.println(N-(Math.abs(zeroC-oneC)));



  }


}

/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