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
   reo012 */

#include <stdio.h>
#include <stdlib.h>

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

//リストにデータを追加
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);
    //元のリストの前に追加する
    new->elem = elem;
    new->next = *first;
    *first = new;
    return;
}

//リストからデータを削除する
void pop(node **first){
    node *tmp = *first;
    if(tmp == NULL) exit(1);
    *first = tmp->next;
    //取り出したノードのメモリを解放
    free(tmp);
    return;
}

int main(void){
    char input_s[100001];
    int i, count=0;
    //最初のノードの指す先
    node *first = NULL;

    //入力
    scanf("%s", input_s);

    //入力された文字列の情報をリストに順に追加
    //ただし、i-1回目に追加されたキューブがi回目のキューブと異なるなら、i-1回目をリストから削除
    for(i=0; input_s[i]!='\0'; i++){
        if(first==NULL || input_s[i]==first->elem){ //最初のキューブ・一個前と同じキューブ
            push(&first, input_s[i]);   //リストに追加
        }else{  //一個前のキューブと異なるとき
            pop(&first);    //一個前のキューブ（リストの先頭にある）を削除
            count += 2;     //countを2増やす
        }
    }
    //答えのcountを出力
    printf("%d\n", count);
    return 0;
}