S = input().strip()

sum_map = {0: -1}
current_sum = 0
max_len = 0

for i, c in enumerate(S):
    current_sum += 1 if c == 'A' else -1
    if current_sum in sum_map:
        current_len = i - sum_map[current_sum]
        if current_len > max_len:
            max_len = current_len
    else:
        sum_map[current_sum] = i

print(max_len)