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.Linq;
using System.Text;
using System.Threading.Tasks;

namespace AtCorder
{
	public class Program
	{
		static void Main(string[] args)
		{
			C();
		}

		public static void C()
		{
			Scanner cin = new Scanner();
			string s = cin.Next();
			List<int> v = new List<int>();
			for (int n = 0; n < s.Length; ++n) {
				v.Add(s[n] == '0' ? 0 : 1);
			}

			int count = Hoge(s, 0);
			Console.WriteLine(count);
		}

		public static int Hoge(string s, int count)
		{
			int n = s.Length - 1;
			if (n <= 0) {
				return count;
			}

			int maxCount = count;
			Parallel.For(0, maxCount, m => {
				if (s[m] != s[m + 1]) {
					int count2 = count + 2;
					string s2 = s.Remove(m, 2);
					int count3 = Hoge(s2, count2);

					if (count3 > maxCount) {
						maxCount = count3;
					}
				}
			});

			return maxCount;
		}

		public static int Piyo(List<int> s, int count)
		{
			int n = s.Count - 1;
			if (n <= 0) {
				return count;
			}

			int maxCount = count;
			for (int m = 0; m < n; ++m) {
				var s2 = new List<int>(s);
				if (s2[m] != s2[m + 1]) {
					int count2 = count + 2;
					s2.RemoveRange(m, 2);
					int count3 = Piyo(s2, count2);

					if (count3 > maxCount) {
						maxCount = count3;
					}
				}
			}

			return maxCount;
		}

		
	}

	public class Scanner
	{
		string[] s;
		int i;

		char[] cs = new char[] { ' ' };

		public Scanner()
		{
			s = new string[0];
			i = 0;
		}

		public string Next()
		{
			if (i < s.Length) return s[i++];
			string st = Console.ReadLine();
			while (st == "") st = Console.ReadLine();
			s = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
			if (s.Length == 0) return Next();
			i = 0;
			return s[i++];
		}

		public int NextInt()
		{
			return int.Parse(Next());
		}
		public int[] ArrayInt(int N, int add = 0)
		{
			int[] Array = new int[N];
			for (int i = 0; i < N; i++) {
				Array[i] = NextInt() + add;
			}
			return Array;
		}

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

		public long[] ArrayLong(int N, long add = 0)
		{
			long[] Array = new long[N];
			for (int i = 0; i < N; i++) {
				Array[i] = NextLong() + add;
			}
			return Array;
		}

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


		public double[] ArrayDouble(int N, double add = 0)
		{
			double[] Array = new double[N];
			for (int i = 0; i < N; i++) {
				Array[i] = NextDouble() + add;
			}
			return Array;
		}
	}
}