s = input()
s = '11011010001011'
 
count = 0
 
def str_del(str,index):
    str = str[:index]+str[index+1:]
    return str
 
def remove_cube(string, count):
  count = string.count('10')
  if count > 0:
    new_text = string.replace('10', '', count)
  count = string.count('01')
  if count > 0:
    new_text = string.replace('01', '', count)
  return new_text
 
string = s
can_remove = True
while can_remove:
  search_count = string.count('10')
  if search_count > 0:
    new_text = string.replace('10', '', search_count)
  else:
    search_count = string.count('01')
    if search_count > 0:
      new_text = string.replace('01', '', search_count)

  if new_text != string:
    string = new_text
    count += search_count * 2
    can_remove = True
  else:
    can_remove = False
 
print(count)
