using System;
using System.Linq;
using System.Collections.Generic;
using System.IO;


class Program
{
    public static long Read()
    {
        return long.Parse(Console.ReadLine());
    }

    public static long[] Reads()
    {
        return Console.ReadLine().Split().Select(long.Parse).ToArray();
    }


    public static void Main()
    {
        var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
        Console.SetOut(sw);

        var s = Console.ReadLine();
        var c = s.ToCharArray();
        var l = new List<char>();
        l.AddRange(c);
        long count = 0;
        bool b = false;
        while(!b)
        {
            for (int i = 0; i < l.Count - 1; i++)
            {
        
                if ((l[i] == '0' && l[i + 1] == '1') || (l[i] == '1' && l[i + 1] == '0'))
                {
                    count++;
                    l.RemoveRange(i,2);
                }
            }

            if (l.Count == 0)
                b = true;
            else
            {
                int k = 0;
                for (int i = 0; i < l.Count - 1; i++)
                {
                    if ((l[i] == '0' && l[i + 1] == '1') || (l[i] == '1' && l[i + 1] == '0'))
                        k++;
                }
                if (k == 0)
                    b = true;
            }
        }

        Console.WriteLine(2 * count);


        Console.Out.Flush();
    }
}