import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

import itertools

S = read().rstrip()

S = [1 if x == ord('A') else -1 for x in S]

Scum = itertools.accumulate(S)

left = dict()
left[0] = -1
answer = 0
for i,x in enumerate(Scum):
    if x in left:
        n = i - left[x]
        if answer < n:
            answer = n
    else:
        left[x] = i

print(answer)