from collections import defaultdict

def main():
    S = input()

    cnt = [0] * (len(S) + 1)
    for i, s in enumerate(S):
        if s == 'A':
            cnt[i+1] = cnt[i] + 1
        else:
            cnt[i+1] = cnt[i] - 1

    num = defaultdict(list)
    for i, v in enumerate(cnt):
        num[v].append(i)

    ans = 0
    for v in num.values():
        if len(v) > 1:
            ans = max(ans, max(v) - min(v))
    print(ans)

main()