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: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Numerics;

namespace AtcoderC
{
    class Program
    {
        /*static void Main(string[] args)
        {
        }*/
        //ABC132C
        //全探索法
        /*static void Main()
        {
            BigInteger N = int.Parse(Console.ReadLine());
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int min = input.Min();
            int max = input.Max();
            int countARC = 0;
            int countABC = 0;
            int Answer = 0;
            for (int i = min; i <= max; i++)
            {
                countABC = 0;
                countARC = 0;
                foreach (int n in input)
                {
                    if (n > i)
                    {
                        countARC++;
                    }
                    else
                    {
                        countABC++;
                    }
                }
                if (countABC == countARC)
                {
                    Answer++;
                }
            }
            Console.WriteLine(Answer);
        }*/
        //2回目
        /*static void Main()
        {
            int N = int.Parse(Console.ReadLine());
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            Array.Sort(input);
            int min = input[(N / 2) - 1];
            int max = input[N / 2];
            Console.WriteLine(max - min);
        }*/

        //ABC131C
        //ユーグリッドの互除法の応用
        /*static void Main()
        {
            BigInteger[] input = Console.ReadLine().Split().Select(BigInteger.Parse).ToArray();
            BigInteger A = input[0];
            BigInteger B = input[1];
            BigInteger C = input[2];
            BigInteger D = input[3];
            BigInteger ans = (B / C) + (B / D);           
            //最小公倍数
            var ans2 = C * D / lcm(C, D);
            BigInteger ans3 = B / ans2;
            var BB = B - ans + ans3;
            ans = (A - 1) / C + (A - 1) / D;
            ans3 = (A - 1) / ans2;
            var AA = (A - 1) - ans + ans3;
            Console.WriteLine(BB - AA);
            
        }
        static public BigInteger lcm(BigInteger a, BigInteger b)
        {
            var r = a % b;
            while (r != 0)
            {
                a = b;
                b = r;
                r = a % b;
            }
            return b;
        }*/

        //129C
        //動的計画法
        //TLEになる時間がかかり過ぎる
        /*static void Main()
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int N = input[0];
            int M = input[1];
            List<int> ttt = new List<int>();
            for (int i = 0;i < M;i++)
            {
                var temp = int.Parse(Console.ReadLine());
                ttt.Add(temp);
            }
            int[] dparray = new int[N+2];
            dparray[N] = 1;
            dparray[N - 1] = 1;
            for (int i = N-1; i >= 0; i--)
            {
                if (ttt.Contains(i))
                {
                    dparray[i] = 0;
                }
                dparray[i] = (dparray[i + 1] + dparray[i + 2]) % 1000000007;
            }
            Console.WriteLine(dparray[0]);
        }*/
        //ABC128
        /*static void Main()
        {

        }*/
        //ABC134A
        /*static void Main()
        {
            int r = int.Parse(Console.ReadLine());
            var ans = 3 * r * r;
            Console.WriteLine(ans);
        }*/
        //ABC134B
        /*static void Main()
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int N = input[0];
            int D = input[1];
            int count = 0;
            int ans = N;
            while (true)
            {
                ans = ans - (D * 2 + 1);
                count++;
                if (ans <= 0)
                {
                    break;
                }
            }
            Console.WriteLine(count);
        }*/
        //ABC134C
        /*static void Main()
        {
            int N = int.Parse(Console.ReadLine());
            List<int> array = new List<int>();
            for (int i =0;i<N; i++)
            {
                array.Add(int.Parse(Console.ReadLine()));
            }
            List<int> ans = new List<int>();
            int max = array.Max();
            var temp  = new List<int>();
            foreach (int n in array)
            {
                 temp.Add(n);
            }
            temp.Sort();
            temp.Reverse();
            int second = temp[1];

            foreach (int nn in array)
            {
                if (nn == max)
                {
                    Console.WriteLine(second);
                }
                else
                {
                    Console.WriteLine(max);
                }
            }
        }*/
        //134ABCD
        /*static void Main()
        {
            int N = int.Parse(Console.ReadLine());
            int count = 0;
            int ans = 0;
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            for (int i = 0;i<N;i++)
            {
                for (int j =0;j<N; j++)
                {
                    if ((j+1)%(i+1)==0)
                    {
                        count += input[j];
                    }
                }
                if (count % 2 == input[i])
                {
                    ans++;
                }
                count = 0;
            }
            if (ans != 0)
            {

            }
        }*/
        //AGC036
        /*static void Main()
        {
            long S = long.Parse(Console.ReadLine());
            long X1 = 1000000000;
            long Y1 = 0;
            /*while (true)
            {
                Y1 = S / X1 + S % X1;
            }
            Y1 = S / X1 + S % X1;
            Console.WriteLine("0 0 " + X1 + " 0 0 " + Y1); 
            /*int tempX = 0;
            List<long> ss = new List<long>();
            while (tempX < S)
            {
                var ans = S % tempX;
                if (ans == 0)
                {
                    ss.Add(ans);
                }
                tempX++;
            }
            Console.WriteLine(tempX);*/
        /*long X1 = 0;
        long Y1 = 1;
        while (true)
        {
            X1 = S % Y1;
            if (X1 <= 1000000000)
            {
                break;
            }
            Y1++;
        }
        Console.WriteLine("0 0 "+X1+" 0 0 "+Y1);
    }*/
        //ABC82A
        /*static void Main()
        {
            double[] input = Console.ReadLine().Split().Select(double.Parse).ToArray();
            double a = input[0];
            double b = input[1];
            double sum = (a + b) / 2;
            Console.WriteLine(Math.Ceiling(sum));

        }*/
        //ABC128C
        /*static void Main()
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int N = input[0];
            int M = input[1];
            int ans = 0;
            List<int[]> K = new List<int[]>();
            for (int i = 0;i<M;i++)
            {
                var temp = Console.ReadLine().Split().Select(int.Parse).ToArray();
                K.Add(temp);
            }
            int[] P = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int[] light = new int[M];
            for (int i = 0;i < Math.Pow(2,N);i++)
            {
                var isrow = 0;
                var temp = int.Parse(Convert.ToString(i, 2)).ToString("0000000000");
                for (int j  =0;j < M;j++)
                {
                    var ison = 0;
                    for (int k = 0; k < K[j][0];k++)
                    {
                        var index = K[j][k] - 1;
                        if (temp[9-index] == '1')
                        {
                            ison++;
                        }
                    }
                    if (ison % 2 == P[j])
                    {
                        isrow++;
                    }
                    else
                    {
                        break;
                    }
                }
                if (isrow == M)
                {
                    ans++;
                }
            }
            Console.WriteLine(ans);*/
        /*nt[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
        int a = input[0];
        int b = input[1];
        var temp = a * b % 2;
        if (temp == 0)
        {
            Console.WriteLine("even");
        }
        else
        {
            Console.WriteLine("odd");
        }
    }*/
        //ABC127C 
        /*static void Main()
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int N = input[0];
            int M = input[1];
            List<int[]> gate = new List<int[]>();
            for (int i = 0; i < M; i++)
            {
                var temp = Console.ReadLine().Split().Select(int.Parse).ToArray();
                gate.Add(temp);
            }
            if (M == 1)
            {
                Console.WriteLine(N);

            }
            else
            {
                List<int> card = new List<int>();
                for (int i = gate[0][0]; i <= gate[0][1]; i++)
                {
                    card.Add(i);
                }

                for (int j = 1; j < gate.Count; j++)
                {
                    List<int> ans = new List<int>();
                    var count = 0;
                    for (int i = gate[j][0]; i <= gate[j][1]; i++)
                    {
                        if (card.Contains(i))
                        {
                            count++;
                            ans.Add(i);
                        }
                    }
                    if (count == 0)
                    {
                        break;
                    }
                    card = new List<int>(ans);
                }
                Console.WriteLine(card.Count());
            }
        }*/
        //127C Revenge TLE
        /*static void Main()
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int N = input[0];
            int M = input[1];
            List<int[]> gate = new List<int[]>();
            for (int i = 0; i < M; i++)
            {
                var temp = Console.ReadLine().Split().Select(int.Parse).ToArray();
                gate.Add(temp);
            }
            int Ans = 0;
            for (int i =1;i<=N;i++)
            {
                var count = 0;
                for (int j = 0;j < gate.Count;j++)
                {
                    if (gate[j][0] <= i && i <= gate[j][1])
                    {
                        count++;
                    }  
                }
                if (count == gate.Count)
                {
                    Ans++;
                }
            }
            Console.WriteLine(Ans);
        }*/
        //127C RevengeVer2
        /*static void Main()
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int N = input[0];
            int M = input[1];
            List<int[]> gate = new List<int[]>();
            for (int i = 0; i < M; i++)
            {
                var temp = Console.ReadLine().Split().Select(int.Parse).ToArray();
                gate.Add(temp);
            }
            List<int> Max = new List<int>();
            List<int> Min = new List<int>();
            for (int i =0;i< M;i++)
            {
                Max.Add(gate[i][1]);
                Min.Add(gate[i][0]);
            }
            int max = Max.Min();
            int min  = Min.Max();
            if (min <= max)
            {
                Console.WriteLine(max - min + 1);
            }
            else
            {
                Console.WriteLine("0");
            }
        }*/
        //ABC126C
        /*static void Main()
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int N = input[0];
            int M = input[1];
            double ans = 0.0;
            for (int i = 1; i<= N;i++)
            {
                double tempP = (double)1 / N;
                int point = i;
                while (point<M)
                {
                    tempP = tempP * 1 / 2;
                    point *= 2;
                }
                ans += tempP;
            }
            Console.WriteLine(ans);
        }*/
        //ABC125C ユーグリッド互除法
        /*static void Main()
        {
            int N = int.Parse(Console.ReadLine());
            int[] A = Console.ReadLine().Split().Select(int.Parse).ToArray();
            Array.Sort(A);
            int lcmr, lcml = 0;
            List<int> answer = new List<int>();
            for (int i =0;i<A.Length;i++)
            {
                lcmr = 0;
                for (int x = 0;x < i;x++)
                {
                    if (lcmr - A[x] >= 0)
                    {
                        lcmr = lcm(lcmr, A[x]);
                    }
                    else
                    {
                        lcmr = lcm(A[x], lcmr);
                    }
                }
                lcml = 0;
                for (int y = i+1; y < A.Length; y++)
                {
                    if (lcml - A[y] >= 0)
                    {
                        lcml = lcm(lcml, A[y]);
                    }
                    else
                    {
                        lcml = lcm(A[y],lcml);
                    }
                }
                var tempans = 0;
                if (lcmr > lcml)
                {
                   tempans = lcm(lcmr, lcml);
                }
                else
                {
                   tempans = lcm(lcml, lcmr);
                }
                answer.Add(tempans);
            }
            Console.WriteLine(answer.Max());
        }
        static public int lcm(int a, int b)
        {
            if (b == 0)
            {
                return a;
            }
            var r = a % b;
            while (r != 0)
            {
                a = b;
                b = r;
                r = a % b;
            }
            return b;
        }*/

        //124C
        /*static void Main()
        {
            string inputstr = Console.ReadLine();
            int count = 0;
            if (inputstr[0] == '0')
            {
                for (int i = 0; i < inputstr.Length; i++)
                {
                    if (i % 2 == 0)
                    {
                        if (inputstr[i] == '1')
                        {
                            count++;
                        }
                    }
                    else
                    {
                        if (inputstr[i] == '0')
                        {
                            count++;
                        }
                    }
                }
            }
            else
            {
                for (int i = 0; i < inputstr.Length; i++)
                {
                    if (i % 2 == 1)
                    {
                        if (inputstr[i] == '1')
                        {
                            count++;
                        }
                    }
                    else
                    {
                        if (inputstr[i] == '0')
                        {
                            count++;
                        }
                    }
                }
            }
            Console.WriteLine(count);
        }*/
        //123C 
        /*static void Main()
        {
            double N = double.Parse(Console.ReadLine());
            List<double> city = new List<double>();
            for (int i = 0; i < 5; i++)
            {
                city.Add(double.Parse(Console.ReadLine()));
            }
            var ans = Math.Ceiling(N / city.Min()) + 4;
            Console.WriteLine(ans);
        }*/
        //82C
        /*static void Main()
        {
            double[] input = Console.ReadLine().Split().Select(double.Parse).ToArray();
            double a = input[0];
            double b = input[1];
            double answer = (a + b) / 2;
            Console.WriteLine(Math.Ceiling(answer));
        }*/
        //128B
        /*static void Main()
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int A = input[0];
            int P = input[1];
            var ans = (P + A * 3) / 2;
            Console.WriteLine(ans);
            int a,b
            cin << a << b;
            cout >> (P + A * 3) / 2 >> endl;
        }*/
        //119A
        /*static void Main()
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int A = input[0];
            int B = input[1];
            if (B % A == 0)
            {
                Console.WriteLine(A + B);
            }
            else
            {
                Console.WriteLine(B-A);
            }
        }*/
        //116A
        /*static void Main()
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int A = input[0];
            int B = input[1];
            int C = input[2];
            Console.WriteLine((A*B)/2);
        }*/
        //ABC31B
        /*static void Main()
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int N = int.Parse(Console.ReadLine());
            List<int> array = new List<int>();
            for (int i =0;i<N;i++)
            {
                array.Add(int.Parse(Console.ReadLine()));
            }
            foreach (int a in array)
            {
                if (a < input[0])
                {
                    Console.WriteLine(input[0] - a);
                }
                else if (input[1] < a)
                {
                    Console.WriteLine("-1");
                }
                else
                {
                    Console.WriteLine("0");
                }
            }
        }*/
        //ABC36B
        /*static void Main()
        {
            int N = int.Parse(Console.ReadLine());
            List<string> map = new List<string>();
            for (int i =0;i< N;i++)
            {
                map.Add(Console.ReadLine());
            }
            List<string> ans = new List<string>();
            for(int j = 0; j  < N;j++)
            {
                var temp = "";
                for (int i = N-1; i >= 0; i--)
                {
                    temp += map[i][j];
                }
                ans.Add(temp);
            }
            foreach (string a in ans)
            {
                Console.WriteLine(a);
            }
        }*/
        //ABC40B
        /*static void Main()
        {
            int N = int.Parse(Console.ReadLine());
            int A = 0;
            int temp = N;
            if (N == 1)
            {
                Console.WriteLine("0");
            }
            else
            {
                while (true)
                {
                    double root = Math.Sqrt(N);
                    if (root != (int)root)
                    {
                        N--;
                    }
                    else
                    {
                        A = (int)root;
                        break;
                    }
                }
                List<int> answer = new List<int>();
                bool flag = false;
                while (A > 0)
                {
                    var a = A;
                    while (A * a <= temp)
                    {
                        var ans = temp - (A * a) + Math.Abs(A - a);
                        if (ans == 0)
                        {
                            flag = true;
                            break;
                        }
                        a++;
                        answer.Add(ans);
                    }
                    if (flag == true)
                    {
                        break;
                    }
                    A--;
                }
                if (flag)
                {
                    Console.WriteLine("0");
                }
                else
                {
                    Console.WriteLine(answer.Min());
                }
            }
        }*/
        //ABC122C
        //TLE
        /*static void Main()
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            string S = Console.ReadLine();
            List<int[]> array = new List<int[]>();
            for (int i = 0; i < input[1]; i++)
            {
                array.Add(Console.ReadLine().Split().Select(int.Parse).ToArray());
            }
            List<int> answer = new List<int>();
            foreach (int[] n in array)
            {
                var tempstr = S.Substring(n[0]-1,n[1]-n[0]+1);
                var count = tempstr.Length - tempstr.Replace("AC","").Length;
                var ans = count / 2;
                answer.Add(ans);
            }
            foreach (int a in answer)
            {
                Console.WriteLine(a);
            }
        }*/
        /*static void Main()
        { 
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            string S = Console.ReadLine();
            List<int[]> array = new List<int[]>();
            for (int i = 0; i < input[1]; i++)
            {
                array.Add(Console.ReadLine().Split().Select(int.Parse).ToArray());
            }
            for (int i = 0;i < S.Length-1;i++)
            {
                if (S[i] == 'A')
                {
                    if (S[i + 1] == 'C')
                    {
                        S = S.Replace(S[i],'a');
                    }
                }
            }
            int count = 0;
            List<int> ansstr = new List<int>();
            for (int i = 0; i < S.Length;i++)
            {
                count = 0;
                for (int j = 0;j <= i;j++)
                {
                    if (S[j] == 'a')
                    {
                        count++;
                    }
                }
                ansstr.Add(count);
            }
            List<int> anss = new List<int>();
            for (int i = 0; i < input[1];i++)
            {
                anss.Add(ansstr[array[i][1]-2] - ansstr[array[i][0]-1]);
            }
            foreach (int a in anss)
            {
                Console.WriteLine(a);
            }
        }*/
        //ABC39B
        /*static void Main()
        {
            int X = int.Parse(Console.ReadLine());
            var ans = Math.Sqrt(X);
            var ans2 = Math.Sqrt(ans);
            Console.WriteLine(ans2);
        }*/
        //ABC38B
        /*static void Main()
        {
            int[] input1 = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int[] input2 = Console.ReadLine().Split().Select(int.Parse).ToArray();
            bool flag1 = false;
            foreach (int a in input1)
            {
                foreach (int b in input2)
                {
                    if (a == b)
                    {
                        flag1 = true;
                    }
                }
            }
            if (flag1 == true)
            {
                Console.WriteLine("YES");
            }
            else
            {
                Console.WriteLine("NO");
            }
        }*/
        //ABC35B
        /*static void Main()
        {
            string S = Console.ReadLine();
            int T = int.Parse(Console.ReadLine());
            int[] pos = { 0, 0 };
            foreach (char a in S)
            {

            }
        }*/
        //ABC34B
        /*static void Main()
        {

        }*/
        //技術ProgramContest4Day1
        //A
        /*static void Main()
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();

        }*/
        //ABC135A
        /*static void Main()
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int A = input[0];
            int B = input[1];
            var temp = B - A;
            if (temp % 2 == 0)
            {
                Console.WriteLine(A + (temp/2));
            }
            else
            {
                Console.WriteLine("IMPOSSIBLE");
            }
        }*/
        //ABC135B
        /*static void Main()
        {
            int N = int.Parse(Console.ReadLine());
            int[] P = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int[] AnsN = new int[N];
            for (int i = 0;i <N;i++)
            {
                AnsN[i] = i + 1; 
            }
            int count = 0;
            for(int i = 0;i < N;i++)
            {
                if (P[i] != AnsN[i])
                {
                    count++;
                }
            }
            if (count <= 2)
            {
                Console.WriteLine("YES");
            }
            else
            {
                Console.WriteLine("NO");
            }
        }*/
        //ABC135C
        /*static void Main()
        {
            int N = int.Parse(Console.ReadLine());
            int[] A = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int[] B = Console.ReadLine().Split().Select(int.Parse).ToArray();
            /*int count = 0;
            for (int i = N;i>0;i--)
            {
                var tempA = A[i] + A[i - 1];
                if (tempA <= B[i - 1])
                {
                    A[i - 1] = 0;
                    count += tempA;
                }
                else
                {
                    if (A[i] <= B[i - 1])
                    {
                        A[i - 1] = tempA - B[i - 1];
                        count += B[i - 1];
                    }
                    else
                    {
                        count += B[i - 1]; 
                    }
                }
            }*/
        /*int count = 0;
        for (int i = 0;i < N;i++)
        {
            var tempA = A[i] + A[i + 1];
            if (tempA <= B[i])
            {
                A[i + 1] = 0;
                count += tempA;
            }
            else
            {
                if (B[i] >= A[i])
                {
                    A[i + 1] = A[i + 1] - (B[i] - A[i]);
                    count += B[i];
                }
                else
                {
                    count += B[i]; 
                }
            }
        }
        //このやり方
        int count = 0;
        for (int i = 0; i < N;i++)
        {
            List<int> temp = new List<int>();
            temp.Add(A[i]);
            temp.Add(B[i]);
            count += temp.Min();

        }
        Console.WriteLine(count);
    }*/
        //ABC135D

        //ABC127A
        /*static void Main()
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            string inputstr = Console.ReadLine();
            char[] temp = inputstr.ToCharArray();
            temp[input[1]-1] =  Char.ToLower(temp[input[1] -1]);
            inputstr = new string(temp);
            Console.WriteLine(inputstr);
        }*/
        //ABC35
        /*static void Main()
        {
            string S = Console.ReadLine();
            int T = int.Parse(Console.ReadLine());
            int[] ans = { 0, 0 };
            int count = 0;
            for (int i = 0; i < S.Length;i++)
            {
                if (S[i] == 'U')
                {
                    ans[1]++;
                }
                else if (S[i] == 'D')
                {
                    ans[1]--;
                }
                else if (S[i] == 'R')
                {
                    ans[0]++;
                }
                else if (S[i] == 'L')
                {
                    ans[0]--;
                }
                else
                {
                    count++;
                }
            }
            List<int[]> answer = new List<int[]>();
            answer.Add(ans);
            answer.Add(ans);
            answer.Add(ans);
            answer.Add(ans);
            foreach (int[] n in answer)
            {
                
            }
      
        }*/
        //ABC114A
        /*static void Main()
        {
            int x = int.Parse(Console.ReadLine());
            if (x == 7 || x == 5 || x == 3)
            {
                Console.WriteLine("YES");
            }
            else
            {
                Console.WriteLine("NO");
            }
        }*/
        //ABC121C
        //TLE
        /*static void Main()
        {
            long[] input = Console.ReadLine().Split().Select(long.Parse).ToArray();
            long n = input[0];
            long M = input[1];
            List<long[]> shop = new List<long[]>();
            for (int i = 0;i < n;i++)
            {
                shop.Add(Console.ReadLine().Split().Select(long.Parse) .ToArray());
            }
            List<long[]> answer = new List<long[]>(Sort(shop));
            long price = 0;
            long count = 0;
            foreach (long[] temp in answer)
            {
                var tempcount = count;
                if (count + temp[1] <= M)
                {
                    count += temp[1];
                }
                else
                {
                    count = M;
                }
                price += (count-tempcount) * temp[0];
                if (M<=count)
                {
                    break;
                }
            }
            Console.WriteLine(price);
        }
        static List<long[]> Sort(List<long[]> A)
        {
            for (int i = 0;i < A.Count;i++)
            {
                for (int j = A.Count-1;j >i;j-- )
                {
                    if (A[j][0] < A[j-1][0])
                    {
                        var temp = A[j];
                        A[j] = A[j - 1];
                        A[j - 1] = temp;
                    }
                }
            }
            return A;
        }*/
        //Sortはライブラリを使った方が早い
        /*static void Main()
        {
            long[] input = Console.ReadLine().Split().Select(long.Parse).ToArray();
            long n = input[0];
            long M = input[1];
            List<long[]> shops = new List<long[]>();
            for (int i = 0; i < n; i++)
            {
                shops.Add(Console.ReadLine().Split().Select(long.Parse).ToArray());
            }
            long count = 0;
            long ans = 0;
            foreach (long[] shop in shops.OrderBy(a => a[0]))
            {
                var i = Math.Min(M - count, shop[1]);
                ans += shop[0] * i;
                count += i;
                if (count >= M)
                {
                    break;
                }
            }
            Console.WriteLine(ans);
        }*/
        //120C
        static void Main()
        {
            /*string input = Console.ReadLine();
            int len = input.Length;
            int count = 0;
            while (input.Contains("01") || input.Contains("10"))
            {
                input = input.Replace("01", "").Replace("10", "");
                count += len - input.Length;
                len = input.Length;
            }
            Console.WriteLine(count);*/
            string s = Console.ReadLine();
            int zero = s.Replace("1", "").Length;
            int one = s.Replace("0", "").Length;
            Console.WriteLine((zero < one ? zero : one) * 2);
        }
    }
}

//125C難しい
/*namespace CSharp
{
    class Program
    {
        static IEnumerable<long> EnumerateDivisors(long n)
        {
            for (long i = 1; i * i <= n; i++)
            {
                if (n % i == 0)
                {
                    yield return i;
                    if (n / i != i)
                    {
                        yield return n / i;
                    }
                }
            }
        }

        static void Main(string[] args)
        {
            ReadString();
            long[] A = ReadLongArray();

            long[] divisors = Enumerable.Concat(EnumerateDivisors(A[0]), EnumerateDivisors(A[1]))
                                        .Distinct().ToArray();
            long ans = divisors.Select(x => new { Value = x, Count = A.Count(v => v % x == 0) })
                               .Where(x => x.Count >= A.Length - 1)
                               .OrderByDescending(x => x.Value)
                               .First().Value;
            Console.WriteLine(ans);
        }

        #region Helpers

        static int[] ReadIntArray() => Console.ReadLine().Split().Select(s => int.Parse(s)).ToArray();
        static long[] ReadLongArray() => Console.ReadLine().Split().Select(s => long.Parse(s)).ToArray();

        static int[] ReadLineIntArray(int count) => Enumerable.Repeat(0, count).Select(_ => int.Parse(Console.ReadLine())).ToArray();
        static long[] ReadLineLongArray(int count) => Enumerable.Repeat(0, count).Select(_ => long.Parse(Console.ReadLine())).ToArray();

        static string ReadString() => Console.ReadLine();
        static int ReadInt() => int.Parse(Console.ReadLine());
        static long ReadLong() => long.Parse(Console.ReadLine());

        static void ReadInt(out int first)
        {
            var array = ReadIntArray();
            first = array[0];
        }

        static void ReadInt(out int first, out int second)
        {
            var array = ReadIntArray();
            first = array[0];
            second = array[1];
        }

        static void ReadInt(out int first, out int second, out int third)
        {
            var array = ReadIntArray();
            first = array[0];
            second = array[1];
            third = array[2];
        }

        static void ReadInt(out int first, out int second, out int third, out int fourth)
        {
            var array = ReadIntArray();
            first = array[0];
            second = array[1];
            third = array[2];
            fourth = array[3];
        }

        static void ReadLong(out long first)
        {
            var array = ReadLongArray();
            first = array[0];
        }

        static void ReadLong(out long first, out long second)
        {
            var array = ReadLongArray();
            first = array[0];
            second = array[1];
        }

        static void ReadLong(out long first, out long second, out long third)
        {
            var array = ReadLongArray();
            first = array[0];
            second = array[1];
            third = array[2];
        }

        static void ReadLong(out long first, out long second, out long third, out long fourth)
        {
            var array = ReadLongArray();
            first = array[0];
            second = array[1];
            third = array[2];
            fourth = array[3];
        }

        #endregion
    }
}*/