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 (GCC 9.2.1) (judge verdict: Accepted)

/*  ex9_2
    kosaq   */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define SIZE 100001

typedef struct node{
    char elem;
    struct node *next;
} node;//リストのノード（スタック内）の定義

//スタックにデータを追加
void push(node **first, char elem){
    //追加する分の領域を確保
    node *new = calloc(1, sizeof(node));
    if(new == NULL) exit(1);
    new -> elem = elem;
    new ->next = *first;
    *first = new;
}

//スタックからデータを取り出す
char pop(node **first){
    char elem;
    node *tmp = *first;
    if(tmp == NULL) exit(1);
    elem = tmp -> elem;
    *first = tmp -> next;
    //取り出したノードのメモリを確保
    free(tmp);
    return elem;
}

int main(void){
    char cube_input[SIZE];//入力する配列を定義
    int count = 0;//取り除く個数を定義
    scanf("%s", cube_input);
    //最初のノードはNULL
    node *first = NULL;

    for(int i = 0; i < strlen(cube_input); i++){
        if(first == NULL || cube_input[i] == first -> elem){
            push(&first, cube_input[i]);
        }//NULLのとき、ノードをスタックに追加する。要素が一致するときはノードをスタックに追加する。
        else{
            pop(&first);//異なる場合は取り除き、カウントを２増やす
            count = count + 2;
        }
    }

    printf("%d\n", count);//答えを出力
    return 0;
}