using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using static System.Console;
using static System.Math;

namespace AtCoder {

    public class Program {
        public static void Main (string[] args) {
            new Program ().Solve (new ConsoleInput (Console.In, ' '));
        }

        public void Solve (ConsoleInput cin) {
            // WriteLine ("入力");
            var S = cin.ReadLine;
            int N = S.Length;
            List<int> s = new List<int> ();
            for (int i = 0; i < N; i++) {
                s.Add (int.Parse (S[i].ToString ()));
            }

            int index = 0;
            int count = 0;
            while (true) {
                if (index + 1 >= s.Count) break;
                if (s[index] != s[index + 1]) {
                    count++;
                    s.RemoveAt (index);
                    s.RemoveAt (index + 1);
                    if (index != 0) index--;
                } else {
                    index++;
                }
            }

            WriteLine (count * 2);
        }

    }

    public class ConsoleInput {
        private readonly System.IO.TextReader _stream;
        private char _separator = ' ';
        private Queue<string> inputStream;
        public ConsoleInput (System.IO.TextReader stream, char separator = ' ') {
            this._separator = separator;
            this._stream = stream;
            inputStream = new Queue<string> ();
        }
        public string Read {
            get {
                if (inputStream.Count != 0) return inputStream.Dequeue ();
                string[] tmp = _stream.ReadLine ().Split (_separator);
                for (int i = 0; i < tmp.Length; ++i)
                    inputStream.Enqueue (tmp[i]);
                return inputStream.Dequeue ();
            }
        }
        public string ReadLine { get { return _stream.ReadLine (); } }
        public int ReadInt { get { return int.Parse (Read); } }
        public long ReadLong { get { return long.Parse (Read); } }
        public double ReadDouble { get { return double.Parse (Read); } }
        public string[] ReadStrArray (long N) { var ret = new string[N]; for (long i = 0; i < N; ++i) ret[i] = Read; return ret; }
        public int[] ReadIntArray (long N) { var ret = new int[N]; for (long i = 0; i < N; ++i) ret[i] = ReadInt; return ret; }
        public long[] ReadLongArray (long N) { var ret = new long[N]; for (long i = 0; i < N; ++i) ret[i] = ReadLong; return ret; }
    }
}