import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;

public class Main
{

  public static void main(String[] args) throws IOException
  {
    BufferedReader r = new BufferedReader(new InputStreamReader(System.in), 1);
    PrintWriter out = new PrintWriter(System.out);

    String s;
    String sl[];

    s = r.readLine();
    //sl = s.split(" ");

    int zeroes = 0;
    int ones = 0;
    int n = s.length();

    for(int i = 0; i < n; i++)
    {
      if(s.charAt(i) == '0')
      {
        zeroes++;
      }
      else
      {
        ones++;
      }
    }
    out.println(n - Math.abs(zeroes - ones));
    out.flush();
  }

}
/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