S = input()


count_1 = 0
count_0 = 0

for i in range(len(S)):
  if S[i]=="1":
    count_1 = count_1 + 1
  elif S[i]=="0":
    count_0 = count_0 + 1
    

if count_1 >= count_0:
  print(count_0*2)
elif:
  print(count_1*2)