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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <iostream>
#include <string>
#include <algorithm>
#include <cassert>
#include <limits>
#include <numeric>
#include <type_traits>
#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <bitset>
#include <functional>
#include <utility>
#include <sstream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <complex>
#include <climits>
#include <time.h>

using namespace std;
typedef long long ll;
typedef vector<ll> vl;
typedef vector<string> vs;
typedef pair<ll, ll> P;
#define FOR(i,a,b) for(ll i=(a);i<(b);i++)
#define REP(i,n)  FOR(i,0,n)

ll gcd(ll a, ll b) {
  while (a && b) {
    if (a >= b) a %= b;
    else b %= a;
  }
  return a + b;
}
ll ketasum(ll n) { //桁の数字全ての和を返す 例: 112 -> 4
    ll sum=0;
    if (n<0) return 0;
    while (n>0) {
        sum += n%10;
        n /= 10;
    }
    return sum;
}

typedef pair<ll, ll> pair_t;

bool comp(const pair_t &a, const pair_t &b) {
        return a.first < b.first;
}

void sort2vectors(vector<ll> &av, vector<ll> &bv)
{ // ふたつの配列をまとめてそーと A[2,1] B[3,4] -> 1,4 2,3
        ll n = av.size();
        vector<pair_t> p(n);
        vector<ll> av2(n), bv2(n);
        for (ll i = 0; i < n; i++)
                p[i] = make_pair(av[i], bv[i]);
        sort(p.begin(), p.end(), comp);
        for (ll i = 0; i < n; i++) {
                av2[i] = p[i].first;
                bv2[i] = p[i].second;
        }
        av = av2;
        bv = bv2;
}
ll facctorialMethod(ll k){ //階乗
    ll s;
    if(k > 1){
        s = k*facctorialMethod(k-1);
    } else {
        s = 1;
    }
    return s;
}

map< int64_t, ll > prime_factor(int64_t n) {
  map< int64_t, ll > ret;
  for(int64_t i = 2; i * i <= n; i++) {
    while(n % i == 0) {
      ret[i]++;
      n /= i;
    }
  }
  if(n != 1) ret[n] = 1;
  return ret;
}
// dp[i] := 長さが i の増加部分列として最後尾の要素のとりうる最小値
template<class T> int LIS(vector<T> a,  bool is_strong = true) {
    const T INF = 1<<30; // to be set appropriately
    int n = (int)a.size();
    vector<T> dp(n, INF);
    for (int i = 0; i < n; ++i) {
        if (is_strong) *lower_bound(dp.begin(), dp.end(), a[i]) = a[i];
        else *upper_bound(dp.begin(), dp.end(), a[i]) = a[i];
    }
    return lower_bound(dp.begin(), dp.end(), INF) - dp.begin();
}
long long modpow(long long a, long long n, long long mod) {
    long long res = 1;
    while (n > 0) {
        if (n & 1) res = res * a % mod;
        a = a * a % mod;
        n >>= 1;
    }
    return res;
}
void foreach_permutation(ll n, std::function<void(ll *)> f) {
  ll indexes[n];
  for (ll i = 0; i < n; i++) indexes[i] = i;
  do {
    f(indexes);
  } while (std::next_permutation(indexes, indexes + n));
}

long double dist(ll x, ll y, ll x2, ll y2){
    return sqrt((x-x2)*(x-x2) + (y-y2)*(y-y2));
}
ll GetDigit(ll num){
    ll digit=0;
    while(num!=0){
        num /= 10;
        digit++;
    }
    return digit;
}
const ll MOD = 1000000007;
const ll INF = 1e15;
const int MAX = 10000000;

long long fac[MAX], finv[MAX], inv[MAX];

// テーブルを作る前処理
void COMinit() {
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for (int i = 2; i < MAX; i++){
        fac[i] = fac[i - 1] * i % MOD;
        inv[i] = MOD - inv[MOD%i] * (MOD / i) % MOD;
        finv[i] = finv[i - 1] * inv[i] % MOD;
    }
}

// 二項係数計算
long long COM(int n, int k){
    if (n < k) return 0;
    if (n < 0 || k < 0) return 0;
    return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}
// Sparse Table
template<class MeetSemiLattice> struct SparseTable {
    vector<vector<MeetSemiLattice> > dat;
    vector<int> height;
    
    SparseTable() { }
    SparseTable(const vector<MeetSemiLattice> &vec) { init(vec); }
    void init(const vector<MeetSemiLattice> &vec) {
        int n = (int)vec.size(), h = 0;
        while ((1<<h) < n) ++h;
        dat.assign(h, vector<MeetSemiLattice>(1<<h));
        height.assign(n+1, 0);
        for (int i = 2; i <= n; i++) height[i] = height[i>>1]+1;
        for (int i = 0; i < n; ++i) dat[0][i] = vec[i];
        for (int i = 1; i < h; ++i)
            for (int j = 0; j < n; ++j)
                dat[i][j] = min(dat[i-1][j], dat[i-1][min(j+(1<<(i-1)),n-1)]);
    }
    
    MeetSemiLattice get(int a, int b) {
        return min(dat[height[b-a]][a], dat[height[b-a]][b-(1<<height[b-a])]);
    }
};


// Suffix Array ( Manber&Myers: O(n (logn)^2) )
struct SuffixArray {
    string str;
    vector<int> sa;         // sa[i] : the starting index of the i-th smallest suffix (i = 0, 1, ..., n)
    vector<int> lcp;        // lcp[i]: the lcp of sa[i] and sa[i+1] (i = 0, 1, ..., n-1)
    inline int& operator [] (int i) {return sa[i];}
    
    SuffixArray(const string& str_) : str(str_) { buildSA(); calcLCP(); }
    void init(const string& str_) { str = str_; buildSA(); calcLCP(); }
    
    // build SA
    vector<int> rank_sa, tmp_rank_sa;
    struct CompareSA {
        int n, k;
        const vector<int> &rank;
        CompareSA(int n, int k, const vector<int> &rank_sa) : n(n), k(k), rank(rank_sa) {}
        bool operator()(int i, int j) {
            if (rank[i] != rank[j]) return (rank[i] < rank[j]);
            else {
                int rank_ik = (i + k <= n ? rank[i + k] : -1);
                int rank_jk = (j + k <= n ? rank[j + k] : -1);
                return (rank_ik < rank_jk);
            }
        }
    };
    void buildSA() {
        int n = (int)str.size();
        sa.resize(n+1), lcp.resize(n+1), rank_sa.resize(n+1), tmp_rank_sa.resize(n+1);
        for (int i = 0; i < n; ++i) sa[i] = i, rank_sa[i] = (int)str[i];
        sa[n] = n, rank_sa[n] = -1;
        for (int k = 1; k <= n; k *= 2) {
            CompareSA csa(n, k, rank_sa);
            sort(sa.begin(), sa.end(), csa);
            tmp_rank_sa[sa[0]] = 0;
            for (int i = 1; i <= n; ++i) {
                tmp_rank_sa[sa[i]] = tmp_rank_sa[sa[i - 1]];
                if (csa(sa[i - 1], sa[i])) ++tmp_rank_sa[sa[i]];
            }
            for (int i = 0; i <= n; ++i) rank_sa[i] = tmp_rank_sa[i];
        }
    }
    vector<int> rsa;
    SparseTable<int> st;
    void calcLCP() {
        int n = (int)str.size();
        rsa.resize(n+1);
        for (int i = 0; i <= n; ++i) rsa[sa[i]] = i;
        lcp.resize(n+1);
        lcp[0] = 0;
        int cur = 0;
        for (int i = 0; i < n; ++i) {
            int pi = sa[rsa[i] - 1];
            if (cur > 0) --cur;
            for (; pi + cur < n && i + cur < n; ++cur) {
                if (str[pi + cur] != str[i + cur]) break;
            }
            lcp[rsa[i] - 1] = cur;
        }
        st.init(lcp);
    }
    
    // calc lcp
    int getLCP(int a, int b) {          // lcp of str.sutstr(a) and str.substr(b)
        return st.get(min(rsa[a], rsa[b]), max(rsa[a], rsa[b]));
    }
};

template< unsigned mod >
struct RollingHash {
  vector< unsigned > hashed, power;

  inline unsigned mul(unsigned a, unsigned b) const {
    unsigned long long x = (unsigned long long) a * b;
    unsigned xh = (unsigned) (x >> 32), xl = (unsigned) x, d, m;
    asm("divl %4; \n\t" : "=a" (d), "=d" (m) : "d" (xh), "a" (xl), "r" (mod));
    return m;
  }

  RollingHash(const string &s, unsigned base = 10007) {
    int sz = (int) s.size();
    hashed.assign(sz + 1, 0);
    power.assign(sz + 1, 0);
    power[0] = 1;
    for(int i = 0; i < sz; i++) {
      power[i + 1] = mul(power[i], base);
      hashed[i + 1] = mul(hashed[i], base) + s[i];
      if(hashed[i + 1] >= mod) hashed[i + 1] -= mod;
    }
  }

  unsigned get(int l, int r) const {
    unsigned ret = hashed[r] + mod - mul(hashed[l], power[r - l]);
    if(ret >= mod) ret -= mod;
    return ret;
  }

  unsigned connect(unsigned h1, int h2, int h2len) const {
    unsigned ret = mul(h1, power[h2len]) + h2;
    if(ret >= mod) ret -= mod;
    return ret;
  }

  int LCP(const RollingHash< mod > &b, int l1, int r1, int l2, int r2) {
    int len = min(r1 - l1, r2 - l2);
    int low = -1, high = len + 1;
    while(high - low > 1) {
      int mid = (low + high) / 2;
      if(get(l1, l1 + mid) == b.get(l2, l2 + mid)) low = mid;
      else high = mid;
    }
    return (low);
  }
};
using RH = RollingHash< 1000000007 >;
void UFinit(ll n);
ll UFfind(ll x);
void UFunite(ll x, ll y);
bool UFsame(ll x, ll y);
ll MAX_N; //Union-Find木使う時はcinする
const double PI = 3.14159265358979;
/******変数定義start(vectorはresizeしようね)******/
string S;
/******変数定義end********************************/
void solve(){
    cin >> S;
    ll C0 = 0;
    ll C1 = 0;
    REP(i, S.size()){
        if(S.substr(i,1) == "0"){
            C0++;
        }else{
            C1++;
        }
    }
    cout << 2*min(C0, C1) << endl;
}

vl UFpar(MAX_N);
vl UFrank(MAX_N);
void UFinit(ll n){
    REP(i, n){
        UFpar[i] = i;
        UFrank[i] = 0;
    }
}
ll UFfind(ll x){
    if(UFpar[x] == x){
        return x;
    }else{
        return UFpar[x] = UFfind(UFpar[x]);
    }
}
void UFunite(ll x, ll y){
    x = UFfind(x);
    y = UFfind(y);
    if(x == y){return;}
    if(UFrank[x] < UFrank[y]){
        UFpar[x] = y;
    }else{
        UFpar[y] = x;
        if(UFrank[x] == UFrank[y]){UFrank[x]++;}
    }
}
bool UFsame(ll x, ll y){
    return UFfind(x) == UFfind(y);
}

int main(){

    //clock_t start = clock();

    cout << std::fixed << std::setprecision(20);
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    solve();
    //clock_t end = clock();
    //const double time = static_cast<double>(end - start) / CLOCKS_PER_SEC * 1000.0;
    //printf("time %lf[ms]\n", time);
}