#include <stdlib.h>
#include<stdio.h>
struct CELL {
	struct CELL *next;
	int		value;
} header;
 
struct CELL *nthp(int n)
{
	struct CELL *q;
	q = &header;
	int i;
	for (i = 0; i < n; i++)
	{
		q = q->next;
	}
	return q;
}
 
void insert(int n, int value)
{
	struct CELL *new, *q;
	q = nthp(n-1);
	if ((new = malloc(sizeof(struct CELL))) == NULL)
		printf("���������m�ۂł��܂���");
 
	new->value = value;
	new->next = q->next;
	q->next = new;
}
 
void add(int value)
{
	insert(0, value);
}
 
void delete_p(struct CELL *q)
{
	struct CELL *p;
	p = q->next;
	q->next = p->next;
	free(p);
}
 
int tumiki(void){
  struct CELL *p;
  p = &header;
  int count=0;
  int check=1;
  while(check==1){
    check=0;
    while(p->next->next!=NULL){
      if(p->next->value==p->next->next->value){
        delete_p(p->next);
        delete_p(p->next);
        count=count+2;
        check=1;
      }
      else
        p=p->next;
    }
  }
}
 
int main()
{
  char *s;
  //scanf("%s",s);
  s="0011";
  int i=0;
  header.next=NULL;
  while(s[i]!='\0'){
  	if(s[i]=='0')
      add(0);
    else
      add (1);
  }
  int a=tumiki();
  printf("%d",a);
  return 0;
}