S = input()
N = len(S)
cnt = [0] * (N + 1)
farthest = {0: 0}

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

    if cnt[i] not in farthest.keys():
        farthest[cnt[i]] = i
    else:
        ans = max(ans, i - farthest[cnt[i]])

print(ans)