import sys
from collections import defaultdict
from itertools import accumulate


def input(): return sys.stdin.readline().rstrip('\n')


def main():
    *s, = accumulate([0] + list(map(lambda x: -1 if x == 'A' else +1, input())))
    a = defaultdict(list)
    for i, v in enumerate(s):
        a[v].append(i)

    ans = 0
    for k, v in a.items():
        ans = max(ans, v[-1] - v[0])

    return ans


if __name__ == '__main__':
    ret = main()
    if ret is not None:
        print(ret)