#include<stdio.h>
#include<string.h>
int main(void){
  char s[100000];
  scnaf("%s", s);
  
  int len;
  len = strlen(s);
  
  int i, j;
  int cnt = 0;
  for(i = 0;i < len; i++){
    if((s[i] == '0' && s[i + 1] == '1') || (s[i] == '1' && s[i + 1] == '0')){
      cnt++;
      for(j = i; j < len; j += 2){
        if(s[j + 2] != '\0'){
          s[j] = s[j + 2];
        }
        else{
          s[j] = '\0';
          break;
        }
        if(s[j + 3] != '\0'){
          s[j + 1] = s[j + 3];
        }
        else{
          s[j + 1] = '\0';
          break;
        }
      }
      i--;
      len -= 2;
    }
  }
  
  printf("%d\n", cnt * 2);
  return 0;
}