# -*- coding: utf-8 -*-
s = input()
s = list(s)
x = 0
maxIndex = len(s) - 1
count = 0
while True:
    if len(s) == x + 1:
        break
    if s[x] != s[x + 1]:
        s.pop(x)
        s.pop(x)
        count += 1
        x -= 1
    else:
        x += 1
print("{}".format(count * 2))
