Problem:
Score : 300 points

Problem Statement

There are N cubes stacked vertically on a desk.

You are given a string S of length N. The color of the i-th cube from the bottom is red if the i-th character in S is 0, and blue if that character is 1.

You can perform the following operation any number of times: choose a red cube and a blue cube that are adjacent, and remove them. Here, the cubes that were stacked on the removed cubes will fall down onto the object below them.

At most how many cubes can be removed?

Constraints

1 \leq N \leq 10^5

|S| = N

Each character in S is 0 or 1.

Input

Input is given from Standard Input in the following format:

S

Output

Print the maximum number of cubes that can be removed.

Sample Input 1

0011

Sample Output 1

4

All four cubes can be removed, by performing the operation as follows:

Remove the second and third cubes from the bottom. Then, the fourth cube drops onto the first cube.

Remove the first and second cubes from the bottom.

Sample Input 2

11011010001011

Sample Output 2

12

Sample Input 3

0

Sample Output 3

0

Solution in C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

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

namespace Contest
{
    class Scanner
    {
        public Scanner()
        {
            _stream = new StreamReader(Console.OpenStandardInput());
            _pos = 0;
            _line = new string[0];
            _separator = ' ';
        }

        private char _separator;
        private StreamReader _stream;
        private int _pos;
        private string[] _line;

        #region get a element

        public string Next()
        {
            if (_pos >= _line.Length)
            {
                _line = _stream.ReadLine().Split(_separator);
                _pos = 0;
            }

            return _line[_pos++];
        }

        public int NextInt()
        {
            return int.Parse(Next());
        }

        public long NextLong()
        {
            return long.Parse(Next());
        }

        public double NextDouble()
        {
            return double.Parse(Next());
        }

        #endregion

        #region convert array

        private int[] ToIntArray(string[] array)
        {
            var result = new int[array.Length];
            for (int i = 0; i < array.Length; i++)
            {
                result[i] = int.Parse(array[i]);
            }

            return result;
        }

        private long[] ToLongArray(string[] array)
        {
            var result = new long[array.Length];
            for (int i = 0; i < array.Length; i++)
            {
                result[i] = long.Parse(array[i]);
            }

            return result;
        }

        private double[] ToDoubleArray(string[] array)
        {
            var result = new double[array.Length];
            for (int i = 0; i < array.Length; i++)
            {
                result[i] = double.Parse(array[i]);
            }

            return result;
        }

        #endregion

        #region get row elements

        #region get array

        public string[] Array()
        {
            if (_pos >= _line.Length)
                _line = _stream.ReadLine().Split(_separator);

            _pos = _line.Length;
            return _line;
        }

        public int[] IntArray()
        {
            return ToIntArray(Array());
        }

        public long[] LongArray()
        {
            return ToLongArray(Array());
        }

        public double[] DoubleArray()
        {
            return ToDoubleArray(Array());
        }

        #endregion

        #region get 2~4 elements

        public void GetRow(out string a, out string b)
        {
            a = Next();
            b = Next();
        }

        public void GetRow(out string a, out string b, out string c)
        {
            a = Next();
            b = Next();
            c = Next();
        }

        public void GetRow(out string a, out string b, out string c, out string d)
        {
            a = Next();
            b = Next();
            c = Next();
            d = Next();
        }

        public void GetIntRow(out int a, out int b)
        {
            a = NextInt();
            b = NextInt();
        }

        public void GetIntRow(out int a, out int b, out int c)
        {
            a = NextInt();
            b = NextInt();
            c = NextInt();
        }

        public void GetIntRow(out int a, out int b, out int c, out int d)
        {
            a = NextInt();
            b = NextInt();
            c = NextInt();
            d = NextInt();
        }

        public void GetLongRow(out long a, out long b)
        {
            a = NextLong();
            b = NextLong();
        }

        public void GetLongRow(out long a, out long b, out long c)
        {
            a = NextLong();
            b = NextLong();
            c = NextLong();
        }

        public void GetLongRow(out long a, out long b, out long c, out long d)
        {
            a = NextLong();
            b = NextLong();
            c = NextLong();
            d = NextLong();
        }

        public void GetDoubleRow(out double a, out double b)
        {
            a = NextDouble();
            b = NextDouble();
        }

        public void GetDoubleRow(out double a, out double b, out double c)
        {
            a = NextDouble();
            b = NextDouble();
            c = NextDouble();
        }

        public void GetDoubleRow(out double a, out double b, out double c, out double d)
        {
            a = NextDouble();
            b = NextDouble();
            c = NextDouble();
            d = NextDouble();
        }

        #endregion

        #endregion

        #region get 2~4 column elements

        public void GetColumn(int n, out string[] a)
        {
            a = new string[n];
            for (int i = 0; i < n; i++)
            {
                a[i] = Next();
            }
        }

        public void GetColumn(int n, out string[] a, out string[] b)
        {
            a = new string[n];
            b = new string[n];
            for (int i = 0; i < n; i++)
            {
                GetRow(out a[i], out b[i]);
            }
        }

        public void GetColumn(int n, out string[] a, out string[] b, out string[] c)
        {
            a = new string[n];
            b = new string[n];
            c = new string[n];
            for (int i = 0; i < n; i++)
            {
                GetRow(out a[i], out b[i], out c[i]);
            }
        }

        public void GetColumn(int n, out string[] a, out string[] b, out string[] c, out string[] d)
        {
            a = new string[n];
            b = new string[n];
            c = new string[n];
            d = new string[n];
            for (int i = 0; i < n; i++)
            {
                GetRow(out a[i], out b[i], out c[i], out d[i]);
            }
        }

        public void GetIntColumn(int n, out int[] a)
        {
            string[] sa;
            GetColumn(n, out sa);
            a = ToIntArray(sa);
        }

        public void GetIntColumn(int n, out int[] a, out int[] b)
        {
            string[] sa, sb;
            GetColumn(n, out sa, out sb);
            a = ToIntArray(sa);
            b = ToIntArray(sb);
        }

        public void GetIntColumn(int n, out int[] a, out int[] b, out int[] c)
        {
            string[] sa, sb, sc;
            GetColumn(n, out sa, out sb, out sc);
            a = ToIntArray(sa);
            b = ToIntArray(sb);
            c = ToIntArray(sc);
        }

        public void GetIntColumn(int n, out int[] a, out int[] b, out int[] c, out int[] d)
        {
            string[] sa, sb, sc, sd;
            GetColumn(n, out sa, out sb, out sc, out sd);
            a = ToIntArray(sa);
            b = ToIntArray(sb);
            c = ToIntArray(sc);
            d = ToIntArray(sd);
        }

        public void GetLongColumn(int n, out long[] a)
        {
            string[] sa;
            GetColumn(n, out sa);
            a = ToLongArray(sa);
        }

        public void GetLongColumn(int n, out long[] a, out long[] b)
        {
            string[] sa, sb;
            GetColumn(n, out sa, out sb);
            a = ToLongArray(sa);
            b = ToLongArray(sb);
        }

        public void GetLongColumn(int n, out long[] a, out long[] b, out long[] c)
        {
            string[] sa, sb, sc;
            GetColumn(n, out sa, out sb, out sc);
            a = ToLongArray(sa);
            b = ToLongArray(sb);
            c = ToLongArray(sc);
        }

        public void GetLongColumn(int n, out long[] a, out long[] b, out long[] c, out long[] d)
        {
            string[] sa, sb, sc, sd;
            GetColumn(n, out sa, out sb, out sc, out sd);
            a = ToLongArray(sa);
            b = ToLongArray(sb);
            c = ToLongArray(sc);
            d = ToLongArray(sd);
        }

        public void GetDoubleColumn(int n, out double[] a)
        {
            string[] sa;
            GetColumn(n, out sa);
            a = ToDoubleArray(sa);
        }

        public void GetDoubleColumn(int n, out double[] a, out double[] b)
        {
            string[] sa, sb;
            GetColumn(n, out sa, out sb);
            a = ToDoubleArray(sa);
            b = ToDoubleArray(sb);
        }

        public void GetDoubleColumn(int n, out double[] a, out double[] b, out double[] c)
        {
            string[] sa, sb, sc;
            GetColumn(n, out sa, out sb, out sc);
            a = ToDoubleArray(sa);
            b = ToDoubleArray(sb);
            c = ToDoubleArray(sc);
        }

        public void GetDoubleColumn(int n, out double[] a, out double[] b, out double[] c, out double[] d)
        {
            string[] sa, sb, sc, sd;
            GetColumn(n, out sa, out sb, out sc, out sd);
            a = ToDoubleArray(sa);
            b = ToDoubleArray(sb);
            c = ToDoubleArray(sc);
            d = ToDoubleArray(sd);
        }

        #endregion

        #region get matrix

        public string[][] GetMatrix(int h)
        {
            string[][] result = new string[h][];
            for (int i = 0; i < h; i++)
            {
                result[i] = Array();
            }

            return result;

        }

        public int[][] GetIntMatrix(int h)
        {
            int[][] result = new int[h][];
            for (int i = 0; i < h; i++)
            {
                result[i] = IntArray();
            }

            return result;
        }

        public long[][] GetLongMatrix(int h)
        {
            long[][] result = new long[h][];
            for (int i = 0; i < h; i++)
            {
                result[i] = LongArray();
            }

            return result;
        }

        public double[][] GetDoubleMatrix(int h)
        {
            double[][] result = new double[h][];
            for (int i = 0; i < h; i++)
            {
                result[i] = DoubleArray();
            }

            return result;
        }

        public char[][] GetCharMatrix(int h)
        {
            char[][] result = new char[h][];
            for (int i = 0; i < h; i++)
            {
                result[i] = Next().ToCharArray();
            }

            return result;
        }

        #endregion
    }

    class Program
    {
        public void Solve()
        {
            var sc = new Scanner();
            string S = sc.Next();
            int cnt = 0;
            int ans = 0;
            HashSet<int>hs = new HashSet<int>();
            for (int i = 0; i < S.Length; i++)
            {
                if (S[i] == '1')
                {
                    cnt++;
                }
                else if (S[i] == '0')
                {
                    cnt--;
                }

                if (hs.Contains(cnt))
                {
                    ans+=2;
                    hs.Remove(cnt);
                }
                else
                {
                    hs.Add(cnt);
                }
            }
            Console.WriteLine(ans);
        }

        static void Main(string[] args) => new Program().Solve();
    }
}