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.Linq;
using System.Collections.Generic;
using System.Text;

namespace Program
{
    public class ABC120C
    {
        static public void Main(string[] args)
        {
            var S = NextString;
            var stack = new Stack<int>();
            for (int i = 0; i < S.Length; ++i)
            {
                if (stack.Any())
                {
                    if (stack.Peek() != S[i] - '0')
                    {
                        stack.Pop();
                    }
                    else
                    {
                        stack.Push(S[i] - '0');
                    }
                }
                else
                {
                    stack.Push(S[i] - '0');
                }
            }

            Console.WriteLine(S.Length - stack.Count());
        }

        static class Console_
        {
            private static Queue<string> param = new Queue<string>();
            public static string NextString()
            {
                if (param.Count == 0) foreach (var item in Console.ReadLine().Split(' ')) param.Enqueue(item);
                return param.Dequeue();
            }
        }
        static int NextInt => int.Parse(Console_.NextString());
        static List<int> NextIntList(int N)
        {
            var ret = new List<int>(N);
            for (int i = 0; i < N; ++i) ret.Add(NextInt);
            return ret;
        }
        static long NextLong => long.Parse(Console_.NextString());
        static List<long> NextLongList(int N)
        {
            var ret = new List<long>(N);
            for (int i = 0; i < N; ++i) ret.Add(NextLong);
            return ret;
        }
        static string NextString => Console_.NextString();
        static void Sort<T>(List<T> l) where T : IComparable => l.Sort();
        static void RevSort<T>(List<T> l) where T : IComparable => l.Sort((x, y) => y.CompareTo(x));
        static IEnumerable<long> Prime(long x)
        {
            if (x < 2) yield break;
            yield return 2;
            var halfx = x / 2;
            var table = new bool[halfx + 1];
            var max = (long)(Math.Sqrt(x) / 2);
            for (long i = 1; i <= max; ++i)
            {
                if (table[i]) continue;
                var add = 2 * i + 1;
                yield return add;
                for (long j = 2 * i * (i + 1); j <= halfx; j += add) table[j] = true;
            }
            for (long i = max + 1; i <= halfx; ++i) if (!table[i]) yield return 2 * i + 1;
        }
        static IEnumerable<long> Divisor(long x)
        {
            if (x < 1) yield break;
            var max = (long)Math.Sqrt(x);
            for (long i = 1; i < max; ++i)
            {
                if (x % i != 0) continue;
                yield return i;
                if (i != x / i) yield return x / i;
            }
        }
        static long GCD(long a, long b)
        {
            long tmpa = a, tmpb = b;
            while (tmpb > 0) { var tmp = tmpb; tmpb = tmpa % tmpb; tmpa = tmp; }
            return tmpa;
        }
        class PQ<T> where T : IComparable
        {
            private List<T> h;
            private Comparison<T> c;
            public PQ(int cap, Comparison<T> c, bool asc = true) { h = new List<T>(cap); this.c = asc ? c : (x, y) => c(y, x); }
            public PQ(Comparison<T> c, bool asc = true) { h = new List<T>(); this.c = asc ? c : (x, y) => c(y, x); }
            public PQ(int cap, bool asc = true) : this(cap, (x, y) => x.CompareTo(y), asc) { }
            public PQ(bool asc = true) : this((x, y) => x.CompareTo(y), asc) { }
            public void Push(T v)
            {
                var i = h.Count;
                h.Add(v);
                while (i > 0)
                {
                    var ni = (i - 1) / 2;
                    if (c(v, h[ni]) >= 0) break;
                    h[i] = h[ni]; i = ni;
                }
                h[i] = v;
            }
            public T Peek => h[0];
            public int Count => h.Count;
            public T Pop
            {
                get
                {
                    var r = h[0];
                    var v = h[h.Count - 1];
                    h.RemoveAt(h.Count - 1);
                    if (h.Count == 0) return r;
                    var i = 0;
                    while (i * 2 + 1 < h.Count)
                    {
                        var i1 = i * 2 + 1;
                        var i2 = i * 2 + 2;
                        if (i2 < h.Count && c(h[i1], h[i2]) > 0) i1 = i2;
                        if (c(v, h[i1]) <= 0) break;
                        h[i] = h[i1]; i = i1;
                    }
                    h[i] = v;
                    return r;
                }
                private set { }
            }
        }
        class PQ<TKey, TValue> where TKey : IComparable
        {
            private PQ<Tuple<TKey, TValue>> q;
            public PQ(int cap, Comparison<TKey> c, bool asc = true) { q = new PQ<Tuple<TKey, TValue>>(cap, (x, y) => c(x.Item1, y.Item1), asc); }
            public PQ(Comparison<TKey> c, bool asc = true) { q = new PQ<Tuple<TKey, TValue>>((x, y) => c(x.Item1, y.Item1), asc); }
            public PQ(int cap, bool asc = true) : this(cap, (x, y) => x.CompareTo(y), asc) { }
            public PQ(bool asc = true) : this((x, y) => x.CompareTo(y), asc) { }
            public void Push(TKey k, TValue v) => q.Push(Tuple.Create(k, v));
            public Tuple<TKey, TValue> Peek => q.Peek;
            public int Count => q.Count;
            public Tuple<TKey, TValue> Pop => q.Pop;
        }
        static class Mod
        {
            static public long _mod = 1000000007;
            static public long Add(long x, long y) => (x + y) % _mod;
            static public long Sub(long x, long y) => (x - y) % _mod;
            static public long Multi(long x, long y) => (x * y) % _mod;
            static public long Div(long x, long y) => (x * Inverse(y)) % _mod;
            static public long Pow(long x, long y)
            {
                var a = 1L;
                while (y != 0)
                {
                    if ((y & 1) == 1) a = Mod.Multi(a, x);
                    x = Mod.Multi(x, x);
                    y >>= 1;
                }
                return a;
            }
            static public long Inverse(long x)
            {
                var b = _mod;
                var r = 1L;
                var u = 0L;
                while (b > 0L)
                {
                    var q = x / b;
                    var t = u; u = r - q * u; r = t;
                    t = b; b = x - q * b; x = t;
                }
                return r < 0 ? r + _mod : r;
            }
            static private List<long> _fact = new List<long>();
            static private List<long> _ifact = new List<long>();
            static private void Build(int n)
            {
                if (n >= _fact.Count)
                    for (int i = _fact.Count; i <= n; ++i)
                        if (i == 0L) { _fact.Add(1); _ifact.Add(1); }
                        else { _fact.Add(Mod.Multi(_fact[i - 1], i)); _ifact.Add(Mod.Multi(_ifact[i - 1], Mod.Pow(i, _mod - 2))); }
            }
            static public long Comb(int n, int k)
            {
                Build(n);
                if (n == 0 && k == 0) return 1;
                if (n < k || n < 0) return 0;
                return Mod.Multi(Mod.Multi(_ifact[n - k], _ifact[k]), _fact[n]);
            }
        }
    }
}