s = list(input())
n = len(s)
pos = 0
cnt = 0

while pos != n - 1 and n != 0:
    if s[pos] != s[pos + 1]:
        del s[pos]
        del s[pos]
        pos = 0
        n -= 2
        cnt += 2
    else:
        pos += 1

print(cnt)
