import java.util.*;

class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String S = sc.next();
    int N = S.length();
    ArrayList<Character> al = new ArrayList<>();
    for (int i = 0; i < N; i++) {
      al.add(S.charAt(i));
    }
    int ans = 0;
    int i = 0;
    while (i < al.size() - 1) {
      if (al.get(i) != al.get(i + 1)) {
        al.remove(i);
        al.remove(i);
        ans += 2;
        i = 0;
      } else {
        i++;
      }
    }
    System.out.println(ans);
  }
}
/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