#pragma warning disable

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.Serialization.Formatters.Binary;
using System.Text;
using System.Text.RegularExpressions;
using System.Diagnostics;
using System.Numerics;
using System.Collections;

static class MainClass
{
    public static void Main(string[] args)
    {

        var s = Console.ReadLine();
        var before = '3';
        var count = 0;

        var len = s.Count();
        while (count <= 1000010)
        {
            count++;
            for (int i = 0; i < s.Length - 1; i++)
            {
                if (s[i] != s[i + 1])
                {
                    s = s.Remove(i, 2);
                }

                //if (i == 0)
                //{
                //    before = s[i];
                //}
                //else
                //{

                //    //if (s[i] == '1' && before == '0')
                //    //{
                //    //    s = s.Remove(i - 1, 2).Insert(i - 1, "##");
                //    //    before = 
                //    //}
                //    //else if (s[i] == '0' && before == '1')
                //    //{
                //    //    s = s.Remove(i - 1, 2).Insert(i - 1, "##");

                //    //}
                //}
            }
        }
        Console.WriteLine(len - s.Count());
        Console.ReadLine();
    }

      

    public static int Manhattan(int x1,int x2,int y1,int y2)
    {
        return Math.Abs(x1 - x2) + Math.Abs(y1 - y2);
    }

   


    public static IEnumerable<IEnumerable<T>> Perm<T>(this IEnumerable<T> items, int? k = null)
    {
        if (k == null)
            k = items.Count();

        if (k == 0)
        {
            yield return Enumerable.Empty<T>();
        }
        else
        {
            var i = 0;
            foreach (var x in items)
            {
                var xs = items.Where((_, index) => i != index);
                foreach (var c in Perm(xs, k - 1))
                    yield return c.Before(x);

                i++;
            }
        }
    }

    
    public static IEnumerable<T> Before<T>(this IEnumerable<T> items, T first)
    {
        yield return first;

        foreach (var i in items)
            yield return i;
    }

    public static Dictionary<T,int> ToCountedDictionary<T>(this IEnumerable<T> items)
    {
        var dic = new Dictionary<T, int>();
        foreach (var item in items)
        {
            if (dic.ContainsKey(item))
                dic[item]++;
            else
                dic.Add(item, 1);
        }
        return dic;
    }
  

    public static long Sums(IEnumerable<int> list)
    {
        var sum = 0l;
        foreach (var item in list)
        {
            sum += item;
        }
        return sum;
    }

    public static long f(long n,long b)
    {
        if (n < b)
        {
            return n;
        }
        else
        {
            return f((long)Math.Floor((double)n/b),b) + n % b;
        }
    }

    public static string N(long n,long b)
    {
        var sb = new StringBuilder();
        while (true)
        {
            sb.Append((n % b).ToString());
            n /= b;
            if (n < b)
            {
                sb.Append(n.ToString());
                break;
            }
        }
        return new string( sb.ToString().Reverse().ToArray() );
    }


    public static BigInteger[] factors;
    public static BigInteger[] revFactors;
    public static void SetFactor(int N)
    {
        factors = new BigInteger[N];
        factors[0] = 1;
        revFactors = new BigInteger[N];
        for (int i = 1; i < N; i++)
        {
            factors[i] = factors[i - 1] * i;
            factors[i] %= Mod1e9;
        }
        revFactors[N - 1] = BigInteger.ModPow(factors[N - 1], Mod1e9 - 2, Mod1e9);
        for (int i = N - 2; i >= 0; i--)
        {
            revFactors[i] = revFactors[i + 1] * (i + 1);
            revFactors[i] %= Mod1e9;
        }
    }
    public static BigInteger GetCombination(int a, int b)
    {
        return (((factors[a] * revFactors[b]) % Mod1e9) * revFactors[a - b]) % Mod1e9;
    }

    #region ライブラリ
    public static long ToInt64(this string str) => long.Parse(str);
    public static int ToInt32(this string str) => int.Parse(str);
    public static BigInteger ToBigInteger(this string str) => BigInteger.Parse(str);
    public static List<string> SplittedStringToList(this string str) => str.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).ToList();
    public static List<int> SplittedStringToInt32List(this string str) => str.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).Select(x => int.Parse(x)).ToList();
    public static List<long> SplittedStringToInt64List(this string str) => str.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).Select(x => long.Parse(x)).ToList();
    public static List<BigInteger> SplittedStringToBigInteger(this string str) => str.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).Select(x => BigInteger.Parse(x)).ToList();
    public const int INF = int.MaxValue / 4;
    public const long LONGINF = long.MaxValue / 2;
    public const int Mod1e9 = 1000000007;

    public static void PrintArray(bool[,] array)
    {
        for (int i = 0; i < array.GetLength(0); i++)
        {
            var sb = new StringBuilder();
            for (int j = 0; j < array.GetLength(1); j++)
            {
                sb.Append(array[i, j]).Append(" ");
            }
            Console.WriteLine(sb.ToString());
        }
    }

    #endregion
}

