from itertools import accumulate

def main():
    S = [0] + list(input())

    d = list(accumulate(S, lambda s, c: s + 1 if c == 'A' else s-1))

    l, r = {}, {}

    for i in range(len(d)):
        if d[i] in l:
            r[d[i]] = i
        else:
            l[d[i]] = i

    ans = 0
    for rk, rv in r.items():
        if rk in l:
            span = rv - l[rk]
            if span > ans:
                ans = span
    print(ans)

main()