import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.List;

public class Main {

    public static void main(String[] args) throws Exception {

        InputStreamReader is = new InputStreamReader(System.in);
        BufferedReader reader = new BufferedReader(is);

        List<String> mylist = new ArrayList<>();

        // 標準入力からの値を変数strinputに代入
        String strinput = reader.readLine();

        while (strinput != null) {

            mylist.add(strinput);
            strinput = reader.readLine();

        }

        String arr01[] = mylist.get(0).split(" ");
        String s = arr01[0];
        int l = s.length();

        while (s.indexOf("01") >= 0 || s.indexOf("10") >= 0) {
            //            System.out.println(s);
            s = s.replaceAll("01", "");
            s = s.replaceAll("10", "");

        }
        System.out.println(l - s.length());

    }

}
