# -*- coding: utf-8 -*-

S = input()
cnt = 0

# O(N^2)は不可。
# 010がある時、01と10を取り除くパターンを作る？
s = S
while True:
    c1 = s.count("01")
    c2 = s.count("10")
    if c1 == 0 and c2 == 0:
        break
    elif c1 >= c2:
        s = s.replace("01", "")
        cnt += c1*2
    else:
        s = s.replace("10", "")
        cnt += c2*2

print(cnt)