import sys
sys.set_int_max_str_digits(0)
from itertools import accumulate
from collections import defaultdict
S = input()
N = len(S)
data = []
for s in S:
    if s == 'A':
        data.append(-1)
    else:
        data.append(1)

rui = list(accumulate(data, initial=0))
memo = defaultdict(list)

for i in range(N + 1):
    memo[rui[i]].append(i)

ansl = [0]
for k, v in memo.items():
    if len(v) >= 2:
        ansl.append(v[-1] - v[0])

print(max(ansl))